Singapore - Caroline Wozniacki stormed into the WTA Finals last four on Friday as Maria Sharapova's early exit crowned Serena Williams as year-end world number one for the second season in a row.
Wozniacki battered Petra Kvitova 6-2, 6-3 for her third victory in Singapore, a result that eliminated the Wimbledon champion but handed Agnieszka Radwanska a spot in Saturday's semis.
As the season finale's group phase ends, Williams is relying on already qualified Simona Halep to get her into the semi-finals by winning at least a set against Ana Ivanovic later.
But the 18-time Grand Slam winner can already celebrate ending a season on top of the rankings for a fourth time after Sharapova's bid to unseat her came to an abrupt halt.
The world number two needed to beat Radwanska in straight sets to stay in the tournament but after winning the first set, she agonisingly lost three match points and a tie-breaker in the second.
Sharapova rallied to win the match 7-5, 6-7 (4/7), 6-2, but the 2004 champion exits the tournament after her earlier losses to Wozniacki and Kvitova and now cannot overtake Williams in the rankings.
"I wanted to end this tournament on a win. I'm very proud," the French Open champion said. "It would've been very easy for me to get down on myself.
"I had so many chances being up, having match point, and just saying, 'You know what? I've lost two matches'. Just so easy to just let it go. I didn't, and that's what I wanted for myself today.
"I got the job done. I know I'm not moving forward, but I'm proud of that effort and to finish the year off in this way."
While Sharapova's season is over, Wozniacki's is still very much in play after a win over Kvitova that left her top of White Group as arguably the form player of the tournament.
Super-fit Wozniacki, who is training for next month's New York marathon in lieu of her cancelled wedding to golfer Rory McIlroy, again made all the running as she beat Kvitova in 69 minutes.
"It feels so great. I've been working so hard and it's paying off now," she said.
Sharapova had a 10-2 head-to-head record against Radwanska going into Friday's match and won their last three meetings, but the world number six was unfazed.
Neither player held serve in the first four games but Sharapova eventually edged ahead and took the set.
In the second, she had the crucial victory on her racquet but passed up two match points at 5-1 and another at 5-3 before Radwanska forced the tie-breaker and snatched it 7/4.
However, the Pole looked a spent force in the third set, enabling Sharapova to save face with her only match win in Asia's first staging of the women's season finale.