World number three Simona Halep withdrew from the Sydney International on Tuesday after being struck down with gastroenteritis.
The Romanian top seed was scheduled to face Karolina Pliskova in her opening match, but pulled out citing the illness ahead of next week's Australian Open in Melbourne.
Halep, who had a breakthrough year in 2014, reaching the final at Roland Garros and the Wimbledon semi-finals, was using the Sydney event as her final lead-in to the opening Grand Slam of the year.
She said she became unwell in Shenzhen in China last week where she beat Swiss Timea Baczinszky in the final.
"I got this problem with my stomach during the tournament in China. I feel like I cannot play," Halep said.
"I was waiting till today to see if I'm ready or not, but I really cannot play. It's tough to start the tournament, and I decided to retire because I think it's the best way."
Halep said she expected to recover over the next few days ahead of her fifth tilt at the Australian Open, where she reached the quarter-finals last year.