-LRB- CNN -RRB- -- 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 -LRB- 7-4 -RRB- 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 was n'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 was n'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 . ''