World number four Elena Rybakina withdrew from the US Open on Thursday, sending French qualifier Jessika Ponchet into the third round by walkover.
The Russian-born Kazakh, who won Wimbledon in 2022, had beaten Australian qualifier Destanee Aiava in the first round.
Rybakina issued a statement saying she was forced to withdraw “because of my injuries”.
“I did not want to finish the last Grand Slam of the year this way but I have to listen to my body, and I hope I can close out the remainder of the year strong,” she said.
Rybakina also withdrew from the Paris Olympics, two weeks after she was stunned by Czech Barbora Krejcikova in the semi-finals at Wimbledon.
Rybakina, who had been favored to a second straight title at the All England Club, later said she had contracted acute bronchitis during the tournament.
She also pulled out of the hard court tournament in Toronto before losing her opening match in Cincinnati this month.
Rybakina leads the WTA tour in aces this season with 332 and won titles in Stuttgart, Brisbane and Abu Dhabi.
She was a finalist at the Australian Open in 2023, but the US Open remains the only Grand Slam in which she has never reached the fourth round.
Rybakina had revealed days before the tournament that she had ended a five-year partnership with coach Stefano Vukov, although she shut down questions about that move in the press conference after her win over Aiava.