SINGAPORE (REUTERS) - Maria Sharpova's bid to end the year ranked tennis' No. 1 was left hanging by a thread after she suffered a second straight defeat at the WTA Finals on Thursday.

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

Sharapova still has a slim mathematical chance of finishing top but has to win the title and rely on other results going her way after she also lost her opening match to Caroline Wozniacki.

"It's easy to sit here and say, yeah, I'm tired. I lost two matches," she said. "But that's not the way I feel or the way that I choose to speak. I still have a match ahead of me (against Agnieszka Radwanska), and I will do my best to finish it on a good note.

"That's the only thing I can ask of myself."

Earlier, Wozniacki edged closer to booking her place in the semi-finals with an impressive 7-5 6-3 win over Radwanska.

Showing no signs of fatigue after her exhausting three-set win over Sharapova two days earlier, Wozniacki rebounded from a slow start to over-run the Pole with an ominous display.

Wozniacki plays her final group match against Kvitova on Friday.

Later, Romanian Simona Halep became the first player to secure a place in the semi-finals, after Serena Williams thrashed Eugenie Bouchard 6-1, 6-1.