SINGAPORE: Serena Williams reached the WTA Finals semis and was crowned year-end world number one without picking up a racquet when results went her way at the season finale here on Friday.

The four-time champion was in stark danger of crashing out but she earned a reprieve when Simona Halep, her tormentor in a crushing 6-0, 6-2 defeat earlier this week, won a set against Ana Ivanovic.

And more stars aligned for the 18-time Grand Slam-winner when world number two Maria Sharapova made an early exit -ending her chances of overtaking the American in the year`s final rankings.

Caroline Wozniacki stormed past Petra Kvitova 6-2, 6-3 to set up a semi-final against Williams on Saturday, while Halep will play Agnieszka Radwanska in the last four.

Ivanovic departs despite beating Halep 7-6 (9-7), 3-6, 6-3, while Sharapova also leaves with her head held high after rallying for a 7-5, 6-7 (4-7), 6-2 win over Radwansl(a.

Under complex round-robin calculations, Sharapova needed a straight-sets victory to survive and she came agonisingly close when she passed up three match points in the second set.

While Sharapova`s season is over, Wozniacki`s is still very much in play after a victory over Kvitova that left her as the only player in the tournament to win all three group matches.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.

Following her three-hour slugfest against Sharapova and straight-sets win over Radwanska, the resurgent former world number one has hit a patch of form that has re-announced her as a force in women`s tennis.

Later, Ivanovic fought bitterly for the straight-sets win over Halep that she needed to stay alive, and momentum was with her when she came from behind to win the first set on a tie-breaker.

But the former French Open champion inexplicably missed an open-court volley as she was broken early in the second set, which she lost 6-3 to end her tournament hopes.

But like Sharapova, she gathered herself for an all-out effort in herEnalsetofthe season and she took it 6-3 to seal her 58th victory of the year, more than any other player.

Poland`s Radwanska needed Wozniacki to beat Kvitova to reach the semi-finals a favour for which she promised to take her good friend shopping, on her credit card.

`Yeah, she told me that, too. I hope she`s going to keep her promise,` Wozniacki said. `She owes me a handbag. I think that`s what I`m going to go for.`-AFP