World number four Elena Rybakina withdrew from the French Open on Saturday due to illness, handing an unexpected title boost to defending champion Iga Swiatek.
Rybakina had been due to face Sara Sorribes Tormo of Spain in the third round in the opening match on Court Philippe Chatrier.
"I was not feeling good yesterday and the day before. I didn't sleep last night," said the 23-year-old Wimbledon champion.
"I had fever and a headache and it's difficult to breathe. I tried in the warm-up but I feel it's the right decision to withdraw."
Rybakina had swept into the last 32 without dropping a set.
The Russian-born Kazakh was seen as a potential champion in Paris having arrived at Roland Garros with the prestigious Italian Open clay-court title under her belt.
She had been seeded to face two-time champion Swiatek in the semi-finals.
"I guess with my allergy that my immune system just went down and I picked up something," said Rybakina. "The doctor said there's a virus in Paris."
Rybakina said she will focus on recovery ahead of defending her title at Wimbledon which gets underway on July 3.
"The plan was to play Berlin, Eastbourne, and Wimbledon. There are not many tournaments on grass, but the most important thing is to get healthy again." Sorribes Tormo, ranked 132 in the world, will be playing in the second week of a Grand Slam for the first time.
She will face either Ekaterina Alexandrova of Russia or Beatriz Haddad Maia of Brazil for a place in the quarter-finals.
Later Saturday, world number one Swiatek takes on China's 80th-ranked Wang Xinyu for a place in the last 16.