World number four Maria Sharapova held her nerve in a tense finish to beat top seed Simona Halep and close in on a last-four place at the WTA Finals.
The Russian withstood a Halep fightback to win 6-4 6-4 and move top of the Red Group in Singapore.
US Open champion Flavia Pennetta earlier beat Agnieszka Radwanska 7-6 (7-5) 6-4 to keep her hopes alive.
Pennetta and Sharapova will meet in the final round of Red Group matches on Thursday, with Halep facing Radwanska.
All four players remain in with a chance of securing one of two semi-final places.
World number one Serena Williams is absent from the eight-player field in Singapore as she recovers from injury.
Sharapova shows true grit
Five-time Grand Slam champion Sharapova showed her quality and fighting spirit as she built a commanding lead, and then gathered herself as Halep came battling back.
Sharapova, completing only her second full match since Wimbledon following injuries, led 6-4 5-1 but failed to convert two chances to serve out the match.
Halep had her chance to level on serve, only for Sharapova - after a pep talk from coach Sven Groeneveld - to rediscover her attacking intent and fire away a backhand on match point.
"I'm quite pleased to be able to beat the number two player in the world," said the 28-year-old, who took her career record against Halep to 6-0.
"I don't think that I played unbelievable tennis today. I still made errors, but cut down on my errors from the first match."
Pennetta keeps hopes alive
The Italian, 33, knew that a second defeat of the week would see her eliminated and it brought out her best performance since winning a first Grand Slam title in New York last month.
Pennetta, who could play her last ever match on Thursday before she retires, fought back from 5-3 down in the first set to win in one hour and 40 minutes.
"Of course it was important today to win this match, because like this I still have a little bit of chance, no?" said Pennetta.
Radwanska said: "Definitely the first set I should have won. The concentration was slipping away sometimes and I wasn't focused enough."
|WTA Finals groups|
|Red Group||White Group|
|Simona Halep (Romania, rank 2)||Garbine Muguruza (Spain, 3)|
|Maria Sharapova (Russia, 4)||Petra Kvitova (Czech Republic, 5)|
|Agnieszka Radwanksa (Poland, 6)||Angelique Kerber (Germany, 7)|
|Flavia Pennetta (Italy, 8)||Lucie Safarova (Czech Republic, 9)|