Serena Williams was on the verge of clinching the year-end world number one ranking for the fourth time yesterday after Maria Sharapova, the only woman with any chance of overtaking her, lost for the second time at the WTA Finals.

Sharapova was beaten 6-3 6-2 by Wimbledon champion Petra Kvitova, leaving her with only a slim chance of leapfrogging Williams for the top spot.

The Russian needs to win the tournament and rely on Williams bowing out before the final to take the top spot, but the odds are now heavily stacked against her.

Even if Sharapova wins her final group match today against Agnieszka Radwanska - who yesterday lost to Caroline Wozniacki 7-5 6-3 – , she needs a slice of luck just to make the semi-finals.

Sharapova would have to beat Radwanska in straight sets and rely on Wozniacki defeating Kvitova in straight sets, but she said she was not looking beyond her final group match.

“Since we don’t play the round robin format very often, my mindset is very similar to any other match: go out and try to win it,” she said.

“I know I lost the previous match, and I wanted to go out and do the best that I could today; just didn’t happen.

“I still have a match ahead of me, and I will do my best to finish it on a good note.

“That’s the only thing I can ask of myself.”

Williams, who previously finished the year ranked number one in the world in 2002, 2009 and 2013, faces an anxious wait before knowing whether she is through to the semis.

The US Open champion thrashed Eugenie Bouchard 6-1 6-1 yesterday to finish the round-robin phase with a 2-1 win-loss record after beating Ana Ivanovic in her opener then losing to Simona Halep.

The only way she can miss out on the semis is if Ivanovic beats Halep in straight sets today.

“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. So whatever happens, happens at this time.”