Maria Sharapova's bid to end the year ranked No.1 was left hanging by a thread after she suffered a second successive defeat at the WTA Finals yesterday.

The only woman capable of overhauling Serena Williams for the top spot, Sharapova was beaten 6-3, 6-2 by the Wimbledon champion Petra Kvitova at Singapore's Indoor Stadium.

Sharapova still has a slim arithmetical chance of finishing No.1 but has to go on and win the title, and she must rely on other results to go her way simply to reach the semis. "It's easy to sit here and say, yeah, I'm tired; I lost two matches," said Sharapova. "But that's not the way I feel or the way that I choose to speak. I still have a match ahead of me, and I will do my best to finish it on a good note."

One day after suffering her heaviest defeat since she was a teenager, Williams was back to her best as she swatted aside Eugenie Bouchard 6-1, 6-1 in her final round-robin match that included a 205km/h ace. The world No.1 had been humiliated 6-0, 6-2 by Romania's Simona Halep on Wednesday but can still make the semis as long as Halep, who is already assured of her place, does not lose her final group match, to Ana Ivanovic, in straight sets.

"Well, my fate really rests on me," said Williams. "If I wanted to win and be a part of the event, I should have won my match yesterday or should have done better."

Caroline Wozniacki edged closer to booking her place in the semi-finals with an impressive 7-5, 6-3 win over Agnieszka Radwanska. Showing no signs of fatigue after her exhausting three-set win over Sharapova two days earlier, Wozniacki rebounded from a slow start to overrun Radwanska with an ominous display.

"I just kept fighting for every point; there was some ridiculous shot-making there at times," said Wozniacki. "I just had to keep going and keep trying to stay close to the baseline, really."

The only way Wozniacki can miss out on the semis is if she loses her final group match to Kvitova in straight sets and Radwanska beats Sharapova in two sets.

The 24-year-old Kvitova had lost her opening match to Radwanska but was in much better form against Sharapova and is excited about her chances of surviving. "I knew that I have a game to beat Maria," she said. "Everything I did today was really good, and I'm glad that I beat her and I have still a chance to go in the semi-finals."

Bouchard became the first player eliminated from the lucrative tournament after she suffered a third straight defeat, but the 20-year-old Canadian was still upbeat. The youngest player to qualify for the elite eight-woman event, Bouchard has won her first WTA title in 2014 and made her first grand slam final, at Wimbledon.

"I feel like I have so much more to learn still and so much more to improve," she said. "Playing against the best in the world today, I got my butt kicked, but I had fun. It actually motivates me to try to get better."