(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.

1. Where is Sharapova from?
2. What Championship was she in?
3. Did she quit?
4. What kind of defeat made her quit?
5. How many wins did Li Na have?
6. When was the last time she won?
7. Who was the U.S. Open winner?
8. When did Sharapova play them?
9. What is Li the first of?
10. What kind of injury did Sharapova have?
11. How many times did she lose in two day?
12. Where were they playing?
13. What is Caroline Wozniacki ranked?
14. Who beat her?
15. In which match?
16. Was it her first?
17. Which one was it?
18. What were the scores?
19. How did Li feel about winning agaist Sharapova?
20. Why?

Numbered answers:
1. Russia
2. WTA Championships
3. Yes
4. lost to China's Li Na
5. 7-6 (7-4) 6-4
6. August
7. Sam Stosur
8. Wednesday
9. first Asian Grand Slam winner
10. ankle injury
11. Two
12. Istanbul
13. World number one
14. Vera Zvonareva
15. red group match
16. No
17. second
18. 6-2-4-6-6-3
19. so happy
20. first match ever at the Championships