(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.
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Maria Sharapova has quit the end-of-season WTA Championships in Istanbul
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The Russian had lost in straight sets to China's Li Na
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It was Sharapova's second straight-sets defeat in two days
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Elsewhere Victoria Azarenka beat U.S. Open Sam Stosur
"I really wasn't thinking about too much during the match," @placeholder was quoted as saying on the WTA's official website after the match.
"I'm so happy I was able to beat @placeholder 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."