NEW YORK — In the latest stunning upset at the U.S. Open, Jessica Pegula took advantage of an off-night for Iga Swiatek and sent the world No. 1 tumbling out of the tournament. The world No. 6 won 6-2, 6-4, in just under 90 minutes, setting up a semifinal against Karolina Muchova of the Czech Republic. She is the fourth American to reach the semifinals of the this year’s tournament in New York, joining Emma Navarro, Frances Tiafoe, and Taylor Fritz.

The last time that multiple American men and women made it to this stage of a Grand Slam tournament was 2003, also at the U.S. Open. Eventual champion Andy Roddick was joined by Andre Agassi, Lindsay Davenport, and Jennifer Capriati.

The win sent the sixth-seeded Pegula into her first Grand Slam semifinal. She had previously lost six quarterfinals, including two in each of the last two seasons, even as her ranking rose to No. 3 in the world and she won WTA Masters 1000 tournaments — just below the level of a Grand Slam.

Approaching her 30th birthday at the end of last season, Pegula fired David Witt, her longtime coach. He had guided her rise from outside the top 100 to within sniffing distance of the top of the sport in a five-year stint. Pegula wanted more. She wanted one of those trophies that Swiatek has been collecting like snow globes.

Now, finally, 15 years after tuning professional, she is two wins away from her first. She capitalised on an error-strewn performance from the world No. 1, which is becoming an unwelcome theme in her rare defeats. When these losses come, Swiatek sprays the ball everywhere except between the lines. She hit 41 unforced errors in 16 games.

This defeat was the five-time Grand Slam champion’s latest stumble in a frustrating summer. She has been on the losing end of battles at every tournament since her triumph at the French Open, which was her fourth title at Roland Garros and her third there in a row. It left no doubt that about her reputation as one of the greatest female clay-court players of the modern era.

Swiatek has been on shaky ground ever since she left her beloved red clay of Paris. She lost in the third round of Wimbledon to Yulia Putintseva, in a defeat that looked like a few other defeats, and a little bit like this one. Even when she briefly returned to clay for the Olympics, she put in a similar performance against Zheng Qinwen in the Olympic semifinals, before losing to Aryna Sabalenka in Cincinnati.

All the bad losses look basically the same. Her serve, which has recently evolved into a more serious weapon, doesn’t land in the court often enough. Her vaunted forehand, a terrifyingly efficient shot in the past three seasons, flies wildly off the court. The backhand, usually a steadying force, starts going off next, and Swiatek, often one of the game’s great problem solvers, just tries to hit the ball even harder, whether it is short in the court or deep on the baseline, expecting a different result that never comes.

Iga Swiatek’s groundstrokes let her down. (Al Bello / Getty Images)

Serving for the match, knowing that Swiatek had pulled rabbits out of her hat and come back from match point down at the French Open, Pegula stayed steady.
She put her serves in and let Swiatek’s errors — including one forehand return that nearly landed outside the tramlines, off a 65 mph second serve — bring her to three match points. Swiatek saved one with a crisp forehand winner across the court, and then another with an even crisper backhand down the line.

But then, like she had all night, she flew a rally backhand wide, and Pegula’s arms were in the air.

(Kena Betancur / AFP via Getty Images)