(CNN) -- Maria Sharapova has pulled out of the season-ending WTA Championships in Istanbul after suffering successive straight-sets defeats. 

The Russian lost to China's Li Na 7-6 (7-4) 6-4, who had herself not won a match, or even a set, since August. 

It was a match that ebbed and flowed as Li came from 2-4 down in the first set to force a tie-break, which she won despite losing the first four points. 

Li, who this year became the first Asian Grand Slam winner after victory in the French Open, then fought off a late Sharapova surge when 5-2 up to take the second set 6-4. The defeat followed Sharapova's capitulation against U.S. Open champion Sam Stosur 6-1, 7-5 on Wednesday. 

"I really wasn't thinking about too much during the match," Li was quoted as saying on the WTA's official website after the match. 

"I'm so happy I was able to beat Maria because she's a top player, and also today was my first match ever at the Championships, so I want to thank all the crowd here for supporting me." 

But for Sharapova, who has struggled with an ankle injury but had the chance to become world number one with victory in Istanbul, there were some positives to be taken from two defeats in two days. 

"I just have to be pleased that I recovered quickly enough to allow myself a chance of playing here," she told AFP. 

World number one Caroline Wozniacki crashed to defeat in her second red group match, losing 6-2 4-6 6-3 to Russia's Vera Zvonareva. 
What Championship was she in?
season-ending WTA Championshi