Serena Williams
Williams ends the year at the WTA as world No.1 for a fourth time this year (Getty Images/Roslan Rahman)

Serena Williams is assured to finish 2014 as World No. 1 after Maria Sharapova crashed out of the season-ending WTA Finals in Singapore.

The Russian required a straight-sets win over Agnieszka Radwanska to keep her hopes alive of usurping Williams at the top of the rankings and of reaching the semi-final.

But despite storming into a 7-5 5-1 lead, Sharapova was unable to close out the match in two sets as Radwanska won a second set tie-break to square the match.

Though Sharapova eventually prevailed 6-2 in the third set to claim her first win of the round robin phase she will miss out on a place in the last four, with Serena Williams now guaranteed to finish as the No. 1 player in the women's game for the fourth time in her career.

Having lost to Wimbledon champion Petra Kvitova on Thursday, Sharapova had put Williams on the brink of the No. 1 ranking, with the American qualifying for the semi-final herself after bouncing back from being thumped by Simona Halep to defeat Genie Bouchard.

Caroline Wozniacki became the third player to qualify for the semi-final virtue of Radwanska denying Sharapova a straight-sets win, with one berth still to be decided for the knock-out phase.