Maria Sharapova came from a set down to beat Svetlana Kuznetsova 5-7 6-2 6-2 in the Sony Ericsson Championships.
The win secured Sharapova's place in the semi-finals and also ensured Ana Ivanovic went through alongside her.
The pair will face off on Friday to see who goes through in first place and avoids world number one Justine Henin.
Meanwhile, Russian 20-year-old Anna Chakvetadze surprised world number three Jelena Jankovic 6-4 0-6 6-3 to take her place in the last four.
Henin, who was already through to the last four, gained some revenge for her shock defeat at Wimbledon by thrashing Marion Bartoli 6-0 6-0.
The WTA's season-finale features the world's top eight players, initially in a two-group round-robin format.
Thursday's win was only Sharapova's third match since the US Open following a recurring shoulder injury.
"I thought I did a good job in the first set, but the fact that I've got less match experience than her right now hurt me," said Sharapova, who won the prestigious end-of-season event in 2004
"But I got myself together and stayed tough and positive to come through it.
"I'm really happy to be here and no matter what happens now I'm just delighted to be playing again. Physically I'm feeling good and this is doing me the world of good."
Defending champion Henin took just 57 minutes to overwhelm Bartoli, who had beaten the world number one in the semi-finals of Wimbledon this year.
It was the heaviest ever defeat in the 35-year history of the end-of-season Championships and ensured Henin topped the yellow group.
"It was a great one, almost everything was perfect," said Henin. "I showed lots of determination and wanted to get my revenge." BBC