Serena Williams was on the verge of clinching the year-end world number one ranking for the fourth time on Thursday 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 at Singapore's Indoor Stadium, leaving her with only the slimmest mathematical chance of leapfrogging Williams for the top spot.
At the every least, the Russian needs to reach the final and rely on Williams bowing out before the final to take the stop spot, but the odds are now heavily stacked against her.
Even if Sharapova wins her final group match against Agnieszka Radwanska, she needs other results to go her way just to make the semi-finals.
Williams previously finished the year ranked number one in the world in 2002, 2009 and 2013.
The reigning U.S. Open champion was due to play Genie Bouchard on Thursday night.
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