Singapore - Serena Williams clinched the year-end world number one ranking for the fourth time on Thursday when Maria Sharapova, the only woman with any chance of overtaking her, lost for a second time at the WTA Finals.
Sharapova only had a slim mathematical chance of leapfrogging Williams for the top spot and needed a strong showing at the WTA Finals.
But the Russian's chances of finishing number one for the first time in her career ended when she was beaten 6-3 6-2 by Wimbledon champion Petra Kvitova at Singapore's Indoor Stadium.
Sharapova also lost her opening match against Caroline Wozniacki and although she cannot reach the number one ranking spot, she still has a chance of qualifying for the semi-finals if she wins her last group match against Agnieszka Radwanska and Wozniacki beats Kvitova.
Williams also finished the year number one in the world in 2002, 2009 and last year. The American won her opening match in Singapore against Ana Ivanovic but was thrashed 6-0 6-2 by Simona Halep in her second match.
The reigning U.S. Open champion was due to play Genie Bouchard on Thursday night. – Reuters