Elena Rybakina, the No. 4 women's player in the world, withdrew from the French Open Saturday due to what was described as an upper respiratory illness.
She was scheduled to face Spain's Sara Sorribes Tormo in the third round.
"I just wanted to give 100%, and obviously I’m far from being 100%," Rybakina said. "If I cannot breathe, there is no chance I can even run and try to compete."
Rybakina is also the reigning Wimbledon champion after defeating Ons Jabeur in three sets in July to win the first Grand Slam of her career.
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The match between Rybakina and Sorribes Tormo in Paris was scheduled to start shortly before noon local time.
Instead, Rybakina was sitting in front of a microphone around the time the match was supposed to begin to explain to reporters why she was dropping out.
Rybakina did sound noticeably congested as she revealed she had difficulty sleeping over the previous two days partially due to a fever.
She also said she experienced issues with her breathing while she was warming up Saturday ahead of a match. She said a doctor told her there is a virus going around "here in Paris."
Rybakina, who won each of her first two matches this week in straight sets, was considered among the top contenders for the championship at Roland Garros. She has won her past 10 matches, including a tuneup title on red clay at the Italian Open last month.
She plays a power game based in part on one of the best serves on the WTA Tour, which she leads in aces this season.
Rybakina is a 23-year-old born in Moscow and has represented Kazakhstan since 2018, when that country offered her funding to support her tennis career.
The switch was a topic of conversation during Wimbledon because the All England Club barred athletes who represent Russia or Belarus from entering the tournament in 2022 due to the war in Ukraine.
Rybakina's forfeit allows the 26-year-old Sorribes Tormo to advance to the fourth round of a Grand Slam tournament for the first time in her career.
The Associated Press contributed to this report.
