Denmark's Caroline Wozniacki is certain to end the year as the world No 1 after Maria Sharapova withdrew from the season-ending WTA Championships with an ankle injury on Wednesday.
Russian Sharapova, who began the day as the only player still able to topple Wozniacki from the top of the rankings, quit after losing 7-6 6-4 to China's Li Na, her second round-robin defeat at the tournament.
The 21-year-old Dane could have clinched the accolade for the second consecutive year with victory over Vera Zvonareva in the Sinan Erdem Dome later in the day but Sharapova's withdrawal settled the issue.
"The ankle didn't recover as well as I had hoped after my first match," said former world No 1 Sharapova, who lost to Samantha Stosur on Tuesday.
"It limits my movement a lot and it's not something I really want to risk," she told reporters. "I tried to push it as much as I could but I think it was a little too soon."
Sharapova injured her ankle in Tokyo recently and only just recovered in time to compete in Istanbul. She will be replaced by first alternate Marion Bartoli of France, who will face Victoria Azarenka on Friday.
Wozniacki beat Agnieszka Radwanska of Poland in her first match here on Tuesday. She has spent all but one week as the world No 1 since last October, briefly losing it to Kim Clijsters in February.
