Naomi Osaka, the No. 1 seed at the United States Open and the defending champion, was defeated in the fourth round on Monday by Belinda Bencic of Switzerland, 7-5, 6-4.
Though an upset based on rankings, Bencic, seeded 13th at the Open, has now beaten Osaka three times this year.
Osaka came into the tournament with concerns about a left knee injury and struggled with the pressure of being No. 1 this summer, but seemed to be reaching top form on hardcourts, her favorite surface.
She defeated Coco Gauff, the 15-year-old phenom, in the third round on Saturday in a match more notable for the kindness and leadership Osaka showed to Gauff in the aftermath.
Bencic, 22, a protégé of Martina Hingis, was a teenage phenom herself, reaching the quarterfinals of the 2014 U.S. Open at 17. But she has battled injuries in recent years and has not been back to that round at a Grand Slam event since.
In the quarterfinals, Bencic will face No. 23 Donna Vekic, who defeated Julia Görges, 6-7 (5), 7-5, 6-3. Vekic, 23, will be playing her first Grand Slam quarterfinal. Görges, the No. 26 seed, served for the match and held match points in the second set.
Osaka’s loss came about 15 hours after the men’s No. 1, Novak Djokovic, was eliminated from the tournament when he retired with an injury from his fourth-round match against Stan Wawrinka.
