Learner Tien has done little in 2024 except win. He reeled off a 28-match streak from May to late July, collecting five titles, including his first at the Challenger level. He reached the quarter-finals at the tour event in Winston-Salem. After picking up two more Challenger crowns and another final, the young American opened his NextGen Finals campaign yesterday with a victory over top-50 player Jakub Mensik, 21st on the Elo list.
If you don’t follow prospects, you can be forgiven if you’ve only recently learned the name. Tien is only two weeks removed from his 19th birthday. He opened the year only barely inside the top 500. There were plenty of reasons to expect big things from the young man–a national 18s title at 16, two junior slam finals–but it would have been foolish to predict so much, so soon.
One reason to moderate expectations is simply age. For those not named Alcaraz or Sinner, it takes time to develop into a top player. Only one man under the age of 21–the cutoff for this week’s event in Jeddah–is ranked inside the top 40. Before Tien turned 19 this month, he was the top-ranked 18-year-old in the world, even with a triple-digit number next to his name.
The climb to the top is even more challenging for youngsters who can’t rely on pure power. Mensik, the highest-ranked teenager, is six feet, four inches tall, with weapons that make him seem bigger. Novak Djokovic recently called him “one of the best servers we have in the game.” The Czech has plenty to learn, and he will surely continue to refine his game. But to compete at the top level, he doesn’t have to.
Tien doesn’t have that luxury. He stands five inches shorter than Mensik. While he may have a bit more growth coming, five-eleven is near the bottom edge of what can be managed on the ATP tour. Only 15 members of the top 100 stand less than six feet, and even that list is skewed toward clay-court specialists. Sebastian Baez is the only five-foot-anything ranked above 45th.
The playing styles available to shorter athletes are limited, especially on hard courts. Tien has already demonstrated his mastery of many of those tactics. He can use his left-handedness to swing serve after serve wide, to a righty’s backhand. He is sturdy from the baseline, and you can take that literally: He’s unafraid of claiming territory right up to the line itself, taking advantage of both his quickness and raw speed. Fearless counterpunching has paid dividends for smaller stars from Olivier Rochus to Kei Nishikori to Alex de Minaur. As a lefty, the American has options those men didn’t.
Still, Tien’s transition from the Challenger tour to the big leagues could be rocky. Good defense and well-executed tactics are enough to clean up against top-200 competition. The combination was (just barely) sufficient against Mensik yesterday. But a full-time spot on the ATP tour requires more.
The game plan
For such a middling server, Tien wins a remarkable number of serve points. He ranks among the top quarter of Challenger tour regulars by serve points won, though his number is helped a bit by spending the entire year on hard courts. He does even better–64.6% compared to a tour norm below 62%–when aces and double faults are taken out of the equation. When the returner gets a racket on the ball, only ten players were better on hard courts.
It’s not surprising, then, that Tien excels on return. Among Challenger players with at least ten matches at the level in 2024, only two men–Dalibor Svrcina and his fellow American in Jeddah, Nishesh Basavareddy–topped Learner’s 42.5% clip. Tien is particularly effective converting second-serve return points.
He’s even better–or at least, he has been better this season–with more on the line. His rate of return points won rises to nearly 47% on break point chances, and he’s just as clutch on the other side of the ball. He saved 65.6% of the break points he faced, second at Challenger level to Mikhail Kukushkin. Here, he has already learned how to use the lefty serve, alternately forcing opponents far out wide and sticking them with uncomfortable body serves when he catches them leaning left for the slider.
The overall package is something between those of two other left-handers, Adrian Mannarino and Cam Norrie. Mannarino, also a sub-six-footer, throws the kitchen sink at opponents, keeping them off balance to compensate for his own lack of power. Norrie is considerably taller and has more firepower at his disposal. But he, too, refuses any rhythm to the man across the net. He alternates a loopy forehand with a flat backhand–except when he doesn’t, if you ever think you’ve found a groove.
Tien serves like Mannarino out of necessity. Even if he doesn’t get taller, the American will surely get stronger, so his 90-mile-per-hour first serves from this year’s US Open probably won’t tell the story of his entire career. But at the moment, he relies on angles and variety. Mannarino has overcome his limitations to the tune of a top-20 peak ranking. On the other hand, his playing style (and the comically loose string tension it relies on) is so unique he hardly provides an example to follow.
In the Las Vegas Challenger final back in September, Tien looked particularly like Norrie. Fighting the wind, he spun forehands and zinged backhands, a combination that made it impossible for the bigger-hitting Tristan Boyer to get comfortable. In other settings, though, the youngster is increasingly using his forehand as a (flatter) weapon, building points one sharp angle at a time.
The most instructive element of these comparisons, though, is the way in which the American differs from his fellow lefties. Mannarino reached Tien’s current ranking when he was 21, after more than 250 pro matches. Norrie–who ultimately peaked inside the top ten–played three years of college tennis and didn’t approach the top 100 in the world until he was 22. Tien, by contrast, is clearing all these hurdles on the first try. Deploying a brainy playing style that normally takes years to refine, the American is making it look natural.
The projection
Aside from size and serve speed, Tien’s future looks bright. The 19-year-old has won 61 of 73 matches across all levels this year. Within a few months, he is likely to crack the top 100. At Challenger level, his serve hasn’t held him back: As we’ve seen, he wins more service points than most of his peers, despite gaining fewer free points with the serve itself.
The question, then, is what effect Tien’s attributes have on career trajectory. Everyone wins fewer points at tour level than at Challengers–the competition is better, so it would be weird if it were otherwise. But the ratio isn’t uniform. Mannarino has won about 7% fewer serve points at tour level than he did in hard-court Challenger matches, while Marcos Giron (another sub-six-footer) lost less than 1% in the transition.
These Challenger-to-tour conversions offer some insight into Learner’s future. Since he has played almost all of his pro matches on hard courts, we’re going to calculate something a bit quirky. How do serve and return win rates change from hard-court Challenger matches to all tour-level matches? That’s what we want to know for the 19-year-old: He’ll need to play on all surfaces soon, probably starting in 2025. This transition he’s about to make–how did it go for other players?
The first-pass answer is that pros are able to retain something like their hard-court Challenger serve win percentage, seeing that number drop by 2%. But they lose a lot against tougher competition on return, winning 7.1% fewer return points. The following table shows those numbers (“Conv%”), along with Tien’s career record at hard-court Challengers (“Tien CH”), along with what the conversion factors suggest for his tour-level win rates (“Tien Adj”):
Conv% Tien CH Tien Adj Serve 98.0% 63.3% 62.0% Return 92.9% 42.3% 39.3%
Those are awfully respectable numbers. 62% serve points is marginal for a tour regular, but combined with 39.3% return points, it’s enough. The combination is about what Francisco Cerundolo managed this year, and he’s ranked 30th in the world.
A word of caution: This type of conversion is not suggesting that Tien’s level is the same as Cerundolo’s now. The calculation involves taking each active player’s career records in tour and Challenger main-draw matches. That probably underestimates Tien’s potential, because most men play the majority of their Challenger matches after their 19th birthday. But a player’s career numbers will include their peak, which typically comes much later. At the very least, these numbers suggest Tien could reach Cerundolo’s level (or better) eventually.
The (other) adjustments
That’s just a first-pass number, because we haven’t gotten to height and handedness. Taking those into account does not help Learner’s case.
Lefties, it turns out, have a rougher transition than right-handers do. Here are the serve and return conversion factors, separated by hand:
Lefties Righties Serve 97.3% 98.1% Return 92.1% 93.0%
Not a huge difference, but hey, the margins in tennis are small. I suspect it is slightly harder for left-handers to move up a level for two reasons. First, the less experienced the opponent, the more valuable it is to be unusual, and lefties are certainly that, making up barely one-tenth of the player pool. At tour level, the novelty is gone: ATP regulars generally know how to handle left-handers.
Second, lefties are more likely to get by with what we might call “crafty” tennis, rather than power. (That’s related to the first reason: They’ve reached Challenger level because they’ve outsmarted inexperienced opponents thus far.) Craftiness might be enough against #180 in the world, but against, say, the Hurkacz serve, all craftiness gets you is a few more tuts of approval in the press box.
Whatever the reason, Tien’s left-handedness means we need to update our tour-level forecast:
(L) Conv% Tien CH Tien Adj Serve 97.3% 63.3% 61.6% Return 92.1% 42.3% 39.0%
Not a huge hit, but ~0.4% of total points won is roughly equivalent to four places in the rankings. A small number here ultimately translates to much bigger ones when denominated by tour-level prize money.
And then, size. Here are the conversion factors for players in three height categories: under six feet, from six feet to six-foot-three, and above six-foot-three:
under 6'0 6'0 to 6'3 over 6'3 Serve 97.0% 97.9% 99.0% Return 92.0% 93.4% 92.6%
Again, craftiness doesn’t convert. Players under six feet tall lose the most points between hard-court Challengers and tour level. The tallest players remain almost as effective on serve, while the middle category retains the most of their return effectiveness.
Here’s the Tien update, using the sub-six-feet conversion rates:
(< 6') Conv% Tien CH Tien Adj Serve 97.0% 63.3% 61.4% Return 92.0% 42.3% 38.9%
Not much of a difference from the left-handed numbers, though we keep going down. This is increasingly the profile of a clay-court specialist, and we might be outside the top 40 now.
Of course, Learner is both left-handed and (relatively) small. My mini-study of active players doesn't give us a big enough pool of data to extrapolate from the small group of small lefties. Instead, a back-of-the-envelope combination of the two factors gives us conversion factors of 96.3% for serve and 91.3% for return:
(L&Sm) Conv% Tien CH Tien Adj Serve 96.3% 63.3% 61.0% Return 91.3% 42.3% 38.6%
For the first time, the adjusted versions of Tien's Challenger-level stats are underwater, summing to less than 100%. Winning 61% of service points would rate fourth-worst in the current ATP top 50, just ahead of Sebastian Baez. 38.6% on return is respectable, though not enough to consistently challenge for titles when combined with such a mediocre serve.
The exact numbers are not important: For one thing, we don't have enough recent data to know exactly how size and handedness interact. Maybe it's not quite that bad. Suffice it to say that both lefties and undersized players are more likely to struggle in the transition from Challengers to the full tour. A player who fits both categories should not expect a smooth trip up the ladder.
For Tien to beat these projections, all he has to do is improve more than the average pro does. As noted above, he already has something of an edge: He posted most of his excellent Challenger numbers as an 18-year-old. That's Alcaraz territory. At the same age, Mannarino was struggling at Futures level, and future top-tenner Norrie was headed off to college. If for some reason Tien plays a lot of Challenger matches in 2025, his stats will probably look better, and the tour-level predictions would change as well.
As Learner and his team are undoubtedly aware, those improvements need to center on the serve. The youngster probably already has what it takes to break serve once or twice a set on tour. But without a bigger first-strike weapon, he'll struggle to get those opportunities. Yesterday he withstood Jakub Mensik's event-record 24 aces, winning in a fifth-set tiebreak despite losing 14 more total points than Mensik did. The American played brilliant tennis, yet it took luck and brilliant timing to pull out the victory. For a five-foot-eleven left-hander among the giants of the professional game, it's not the last tightrope he'll have to walk.
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