WITH FORMER champions Novak Djokovic, Andy Murray, and Stan Wawrinka all missing from this year’s US Open, which began on August 28th, a throng of less-familiar names headed to Flushing with a chance to make a name for themselves. Now that the men’s field is down to the final eight, one name is particularly surprising: Diego Schwartzman (pictured, left), a 25-year-old Argentine who is just five feet and seven inches (1.70m) tall. Mr Schwartzman, nicknamed “El Peque” (“the small one”), had only once before reached the third round of a major. He is a specialist on slow clay surfaces, but ahead of his arrival in New York he had won fewer than half of his matches on hard courts.

Mr Schwartzman’s career-best performance caps what has been a breakthrough season. The highlight so far has been a four-set, upset victory in the third round at Flushing over the fifth seed Marin Cilic (pictured, right), who won the tournament in 2014 (and incidentally stands eleven inches taller than the Argentine). On Monte Carlo’s clay in April, the Argentine reached his first quarter-final at the Masters level, the rung just below tennis’s grand slams, before falling in a tight match against Rafael Nadal. A month later he made his first grand-slam third round at the French Open, forcing a fifth set before finally losing to Mr Djokovic. This summer, he battled to another Masters-level quarter-final, this time on a hard court in Montreal. With his result in New York, Mr Schwartzman, who before this year had never been ranked inside the top 50 by the Association of Tennis Professionals (ATP), will break into the top 30.

This is heady territory for a man whose diminutive stature makes him look more like a ballboy than a professional. In tennis, as in most sports, size matters. Mr Schwartzman can take some solace from the fact that bigger is not always better. The game’s most gargantuan male players do have occasional successes: Juan Martín del Potro, who has reached the quarter-finals in Flushing this year, is the same height as Mr Cilic (six feet and six inches) and is also a former US Open champion, having won the tournament in 2009. But no man taller than Marat Safin, a Russian of six feet and four inches who reached the number-one spot briefly in 2000, has ever topped the world rankings. Behemoths tend to have thunderous and sharply-angled serves, which is the most important shot, especially on pacey hard courts. John Isner and Ivo Karlovic, who at six feet and 11 inches are the gangliest members of the top 50, have both thumped down aces on 23% of serves in the last year, a greater share than any of their peers. But such titans are also cumbersome in rallies. Both of those players have won less than 30% of return points in the same period, a rate bettered by all of the remaining 48. That problem may prevent Alexander Zverev, a talented, lanky 20-year-old, from becoming an all-time great.

The bad news for El Peque is that being at the smaller end of the spectrum seems to be even more of a shortcoming. Of the current top-20 male players only two—Kei Nishikori (five feet and ten inches) and David Goffin (five feet and 11 inches)—are less than six-feet tall. The most notable sub-six-footer in recent years has been David Ferrer, a Spaniard with 27 career tour-level titles and a peak ranking of No. 3 in the world. Yet Ferrer has two inches on Mr Schwartzman. Arnaud Clément, one inch taller than the Argentine, reached the Australian Open final in 2001 and briefly cracked the top ten. But when considering players below five feet and eight inches, the pickings are slim. Olivier Rochus, a Belgian standing at five feet and six inches, reached the Wimbledon fourth round in 2003 and achieved a peak ranking of 24th. The last grand-slam quarter-finalist as short as Mr Schwartzman was Peru’s Jaime Yzaga, who reached the US Open’s final eight in 1994, the best performance of his career.

Predictably, Mr Schwartzman is weakest on his serve. He hits the lowest share of aces (2.5%) and wins the lowest share of service games (63%) of any top-50 player. He compensates for this with deadly returning. In fact, so potent is El Peque in rallies that if tennis had evolved with an underhand serve—as is the case with badminton—he would probably have become a hallowed occupant of the Hall of Fame. In 2017, coming into the US Open, he had won 44% of his return points, better than anyone else on tour, and just ahead of Mr Nadal, Mr Murray, and Mr Djokovic. Against first serves, he is third best; facing second serves, he once again leads the pack. He also ranks in the top four—with the same fearsome trio of Mr Nadal, Mr Murray, and Mr Djokovic—in return games won. Mr Cilic was only his latest prominent victim. On average, the Croat wins 66% of his service points, yet against Mr Schwartzman, he claimed a paltry 55%.

Whatever happens in the US Open’s second week, the Argentine has announced himself as much more than a curiosity. Thanks to this year’s high-profile no-shows and a lucky draw—the tournament’s overwhelming favorites, Mr Nadal and Roger Federer, are likely to meet in the top half—he has a chance to exceed Mr Yzaga’s high point. His quarter-final opponent, the 12th-seeded Spaniard Pablo Carreño Busta, is another clay-court specialist with little pedigree on hard surfaces. Big-serving opponents await in the semi-finals: either 17th-seeded American Sam Querrey or 28th-seeded South African Kevin Anderson. Though Mr Schwartzman would not be favored in a fast-court matchup with either player, the US Open’s playing surfaces have ranked among the tour’s slowest hard-courts since last year’s event. El Peque's odds of winning the title are slight: currently around one-in-70, according to Betfair’s exchange. But David does occasionally vanquish Goliath.