World number one Caroline Wozniacki beat Agnieszka Radwanska on day one of the year-ending WTA Championships as Maria Sharapova lost to Sam Stosur.
Wozniacki won 5-7 6-2 6-4 at the event in Istanbul, while world number two Sharapova was beaten 6-1 7-5.
Denmark's Wozniacki now needs to win one more round-robin match and reach the final to finish 2011 in top spot.
Wimbledon champion Petra Kvitova beat Vera Zvonareva 6-2 6-4 with the top two from each pool making the semi-finals.
Wozniacki was drawn with Zvonareva, Radwanska and Kvitova in the red group in Turkey, while Sharapova, US Open winner Stosur, Victoria Azarenka and Li Na complete the line-up in the white group.
Top seed Wozniacki struggled in the opening exchanges of her match and handed Radwanska two break points to gift the first set to the Pole, but the second set featured six breaks of serve, with the Dane getting four of them.
Wozniacki then squandered a 4-2 lead in the third set but broke again when Radwanska netted a forehand on the 21-year-old's second match point.
"I'm very happy to pull through," said Wozniacki. "It was a tough battle out there.
"Agnieszka has been playing very well lately. It was a bit frustrating not to get the first set after having three set points, but I managed to keep my head cool and fight back.
"It was frustrating not being able to get the first set after the set points but I managed to keep my head cool."
Sharapova, who must at least reach the final to have any chance of finishing the year as number one, was playing her first match since pulling out of the quarter-finals in Tokyo with an ankle injury last month.
The Russian failed to produce a single break point against Stosur in the first set and let slip a 3-0 lead in the second.
"It did take me a while to get going," said Sharapova. "It's obviously tough to come back after a month and compete at this level. But the good thing is that I gave myself a chance, and I did everything I could to even be here."
The three-time Grand Slam winner finished with six double faults and only made 59% of her first serves - compared to 74% for Stosur.