So what happened? Well: Federer and Nadal played on (and on), Murray and Novak Djokovic established themselves as world dominators, and tennis’s two-man monopoly reorganized as the “Big 4.” Del Potro injured his wrists, and whatever spaces that remained in the top 25 were largely peopled with workhouse journeymen such as Spain’s David Ferrer, Russia’s Nikolay Davydenko, and Germany’s Tommy Haas. Slowly and seemingly without warning, the elite ranks of men’s tennis evolved from a playground for men-children such as Michael Chang (who won the 1989 French Open at age 17) and Pete Sampras (who won the 1990 U.S. Open at age 19) to something resembling an active-senior living community.

And then, just as surreptitiously, the game got younger. In August, the analytics blog Heavy Topspin conducted a historical census of the Association of Tennis Professionals’ top 50 players . At the end of 2017, the mean age for that cohort was 29.0 years. Entering this year’s U.S. Open, it was 27.75 years, a 35-year low. And that was with eight 30-somethings bringing up the average: among them, Nadal (32), Federer (37), and Djokovic (31), who together have won 50 of the past 60 Grand Slams.

During the first week of the U.S. Open, the youth movement advanced another couple of steps forward. Of the 16 men who advanced to round four, 10 of them were 29 or younger, and three of them were 25 or younger. It was much the same story through 2018’s three previous Grand Slams, starting at the Australian Open—which also saw 10 under-30-somethings crash the fourth round.

There’s reason for this. Djokovic slumped . Murray got hurt . Haas and Davydenko retired. Ferrer, who forfeited his first-round match against Nadal due to an injury, is close to calling it a career. All the while, young players were getting bigger, faster, stronger, and using match films and advanced statistics to guide their training. It was only a matter of time before the Big 4 couldn’t hold them down anymore.

Rising up to challenge those mainstays are giant-slayers such as Austria’s Dominic Thiem (25), the ninth-seeded clay-court specialist who narrowly lost his five-set quarterfinal against Nadal; Nikoloz Basilashvili (26), the unseeded Georgian who gave Nadal a fright in the fourth round; Japan’s Kei Nishikori (28), a revitalized former U.S. Open finalist who made his third career U.S. Open quarterfinals; John Millman (29), the unseeded Aussie who outhit and outlasted Federer to score one of the tournament’s all-time shockers; and Marin Čilić of Croatia (29), a 2016 U.S. Open champion who’s actually broken through on a big stage. The other one, of course, is del Potro.

The big Argentine appears to be on the brink of winning another major in Flushing Meadows. He wouldn’t drop a set through his first three matches and cruised through a sweltering fourth-round night match against the 21-year-old Borna Ćorić of Croatia.