(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. 
Where is Sharapova from?
What Championship was she in?
Did she quit?
What kind of defeat made her quit?
How many wins did Li Na have?
When was the last time she won?
Who was the U.S. Open winner?
When did Sharapova play them?
What is Li the first of?
What kind of injury did Sharapova have?
How many times did she lose in two day?
Where were they playing?
What is Caroline Wozniacki ranked?
Who beat her?
In which match?
Was it her first?
Which one was it?
What were the scores?
How did Li feel about winning agaist Sharapova?
Why?
Russia
 Russian
WTA Championships
season-ending WTA Championshi
Yes
Maria Sharapova has pulled out of the season
lost to China's Li Na
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. 
7-6 (7-4) 6-4
 7-6 (7-4) 6-4,
August
t, since August. 
Sam Stosur
U.S. Open champion Sam Stosur 
Wednesday
on Wednesday.
first Asian Grand Slam winner
he first Asian Grand Slam winner 
ankle injury
ho has struggled with an ankle injury 
Two
there were some positives to be taken from two defeats in two days. 

Istanbul
victory in Istanbul, 
World number one
World number one Caroline Wozniacki c
Vera Zvonareva
 Russia's Vera Zvonareva.
red group match
ed group match
No
second
second
efeat in her second 
6-2-4-6-6-3
 losing 6-2 4-6 6-3
so happy
'm so happy I was able to beat Maria because she's a top player
first match ever at the Championships
because she's a top player, and also today was my first match ever at the Championships