A dramatic day at the BNP Paribas WTA Finals Singapore presented by SC Global saw the four semi-finalists being decided, as Serena Williams, Simona Halep, Agnieszka Radwanska and Caroline Wozniacki advance out of the opening round-robin stage at the Singapore Indoor Stadium on Friday.

Wozniacki stormed into the semi-finals after winning her third straight match, beating world No. 3 Petra Kvitova 6-2, 6-3. World No. 2 Maria Sharapova 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 as the year-end No. 1 player.

"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. 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."

Despite receiving the rankings boost, Williams, who previously finished the year ranked No. 1 in the world in 2002, 2009 and 2013, still faced an anxious wait before discovering that she had advanced to the semi-finals.

Ana Ivanovic needed to beat Halep in straight sets to advance to the semi-finals in place of Williams, but although she won the first-set tie-breaker 9-7, she faltered in the second set, allowing Halep to win 6-3 and Williams to go through. Ivanovic did eventually win the tie 7-6 (9-7), 3-6, 6-3, but failed to advance.