Romania's Simona Halep pulled out of her final warm-up event before the Australian Open on Tuesday, saying she was suffering from gastroenteritis.
The World No 3 told organisers of the Sydney International she was unable to take her place in the tournament after falling ill in China over the weekend.
Halep won the Shenzhen Open at the weekend even though she said she was already feeling sick.
The 23-year-old had hoped to recover before her first scheduled match in Sydney, where she was the top seed, but said she was still unwell.
"During the tournament in China I got this problem in my stomach," she said. "I didn't play for three days. I was waiting until today to see if I was ready to play again, but I feel like I cannot play.
"It's tough to play a tournament like this so unfortunately I decided to withdraw."
Halep, runner-up at last year's French Open, said she expected to recover in time for the Australian Open, starting next Monday, where she is among the favourites.
"You never know with the stomach, but hopefully in two or three days I'll be ready to play again," she said.
"Right now I just need to think about recovering, then I will worry about matches later."
Halep is the second big name to pull out of Sydney in the last two days. On Monday, US Open runner-up Caroline Wozniacki withdrew due to a wrist injury after reaching the final in Auckland on Saturday.
