Maria Sharapova went out of the WTA Finals and missed her chance to end the year as world number one despite a win over Agnieszka Radwanska in Singapore.
The Russian could have kept alive her hopes with a straight-sets win in her final group match, but missed three match points in the second set.
Sharapova went on to win 7-5 6-7 (4-7) 6-2, but the result means Serena Williams retains the top ranking.
Caroline Wozniacki beat Petra Kvitova 6-2 6-3 in their final group match.
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Wozniacki had already guaranteed a place in the semi-finals and she is joined by Radwanska, with Wimbledon champion Kvitova following Sharapova out of the White Group.
Simona Halep took on Ana Ivanovic in the last Red Group contest, with the Romanian already through and Ivanovic able to knock out Williams with a straight-sets victory.
When Ivanovic recovered from 5-2 down to win the first set it appeared Williams might be in danger, but Halep clinched the second to rescue the American before Ivanovic claimed the consolation of a 7-6 (9-7) 3-6 6-3 victory.
Williams will now take on Caroline Wozniacki in the semi-finals on Saturday, while Radwanska faces Halep.
Sharapova had looked certain to clinch the straight-sets win she needed when she led Radwanska by a set and 5-1, but three match points slipped by as the Pole fought back to force a decider.
The Russian eventually prevailed after three hours and nine minutes but it was not enough to extend her season to the semi-finals.
"I got the job done," said the French Open champion. "I know I'm not moving forward, but I'm proud of that effort and to finish the year off on this way.
"I will not be chasing the number one ranking next season. I think adding more Grand Slams is the top priority."
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Radwanska revealed that she had seen Wozniacki during the break between matches, knowing her fate was in the Dane's hands.
"We talked," said Radwanska. "When I saw her, she was already, 'I know, I know.' She knew what was going on. She wasn't surprised.
"I think we are just going to go shopping, but with my credit card."
Wozniacki added: "I hope she's going to keep her promise. She owes me a handbag. I think that's what I'm going to go for."