(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.





In a repeat of last year's semifinal, which Wozniacki won in straight sets, Zvonareva, 27, hit 49 winners to overpower her Danish opponent in two hours and 18 minutes.





"We always have tough matches and this one was very tough too, three sets as usual," Zvonareva said of the pair's first meeting since the final of the Qatar Open in February, which she won 6-4 6-4.





Despite losing, Wozniacki, 21, is now guaranteed to finish the year at the top of the world rankings courtesy of Sharapova's withdrawal.





Elsewhere fourth seed Victoria Azarenka carded a victory in her first match of the Championships with a 6-2 6-2 defeat of U.S. Open champion Sam Stosur.





Azarenka has now won all five of the matches she has played against Australia's Stosur, sealing success on Wednesday in one hour and 17 minutes on her fourth match point.





Stosur, seeded seventh, was broken four times in the match and will need to bounce back quickly in order to repeat her semifinal appearance at last year's tournament.





Azarenka, eliminated at the group stage of the event in 2010, was delighted with her form in the match.





"I feel great," she told the WTA's official website after scoring a sixth straight-sets triumph in a row.





"I wasn't expecting to play so well in my first match here. It was getting close in the end, we had a really close last game.





"Sam started to really go for every shot with nothing to lose, and I backed up a little bit. She made some incredible shots. But I stayed strong. I had to work for it. I'm glad I finished it now."
Who beat U.S. Open Sam Stosur? Who lost in straight sets to China's Li Na? Who has quit the end-of-season? when Maria Sharapova has quit the end-of-season WTA Championships in Istanbul?
Sharapova's Sharapova Sharapova successive straight-sets defeats.
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