Serena Williams secured a place in the last four of the WTA Finals on Friday and clinched the year-end number one ranking for the fourth time in her illustrious career. The American was not involved in Friday’s final group matches but still emerged as the big winner after Maria Sharapova and Ana Ivanovic both bowed out of the tournament despite winning their final matches. Sharapova, who needed to go on and win the season-ending event to overtake Williams at the top of the rankings, had to beat Poland’s Agnieska Radwanska in straight sets to reach the semis but blew her chance after a second set meltdown. 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 on this way Maria Sharapova She squandered three match points and a 5-2 lead to lose the second set in a tiebreak, and although the Russian recovered to win the match 7-5, 6-7(4-7), 6-2, she failed to make it out of the round-robin stage of the elite eight-woman event. "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," said Sharapova. "(It would have been) so easy to just let it go but I didn’t. "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 on this way." Williams, who won two of her three group matches, faced another anxious wait before knowing whether she would play on the weekend. Ivanovic had the chance to leapfrog her in the group standings, but the Serbian needed to beat Simona Halep in straight sets. After coming from 5-2 behind to win the first set in dramatic fashion, Ivanovic had all the momentum but she was unable to finish off Halep in the second, and had to settle for a 7-6(7-6), 3-6, 6-3 consolation victory. "It’s mixed emotions, obviously," said Ivanovic. "I feel like it was such a great match, yet it’s such a low not to be able to qualify for the semi-finals." Williams will now play her good friend Caroline Wozniacki in the semis, in a rematch of last month’s US Open final, after the Dane romped to a 6-2, 6-3 win over Wimbledon champion Petra Kvitova. Wozniacki’s lopsided victory also gifted a place in the semis to Radwanska, who edged out Kvitova and Sharapova on a countback after the trio all finished with 1-2 win-loss records. Wozniacki was already assured of her place in the semis before her match with Kvitova even started but showed no mercy against the Czech as she chalked up another impressive victory. Radwanska said she would take Wozniacki out on a shopping spree to thank her for keeping her in the tournament and Wozniacki joined in the irreverent mood by saying she had already picked out her gift.