Defending champion Naomi Osaka suffered a surprise defeat in the U.S. Open’s fourth round on Monday after losing two straight sets to 13th-seeded Belinda Bencic, a result that follows on the heels of men’s defending champion Novak Djokovic's loss.

Bencic, of Switzerland, outplayed Osaka at Arthur Ashe Stadium in New York, winning 7-5, 7-6. She is now 3-0 against Osaka this season and has a tour-leading nine victories over top-10 opponents in 2019.

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Osaka, of Japan, didn’t appear in top form, wearing a black sleeve on her nagging left knee. A trainer came out to visit the 21-year-old after Bencic went up 3-2 in the second set.

Osaka’s result comes a day after fellow defending champ Djokovic left his fourth-round match against Stan Wawrinka due to a painful left shoulder. Both were No. 1 seeds going into the tournament.

Osaka made her breakthrough at Flushing Meadows a year ago, winning her first major championship by beating Serena Williams in a chaotic final that devolved after Williams got into an extended argument with the chair umpire.

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Bencic, who has been mentored by Martina Hingis, is 4-1 over her career against top-ranked players.

The Associated Press contributed to this report.