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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.