Maria Sharapova fought her way through to the semi-finals of the WTA Finals in Singapore with a straight-sets win which brought an end to the career of US Open champion Flavia Pennetta.

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The third-seeded Russian edged a tight opening set against the Italian, who had previously announced this would be her final year on the women's tour, before running away with the second to seal a 7-5 6-1 win.

"I've lost the last couple of times against Flavia. I know what a tough competitor and opponent she is. I just wanted to get a little bit of revenge before she goes out of the game!" Sharapova told wtatennis.com.

"She's had an incredible career and I was so happy she got the grand slam at the US Open. I was smiling for her."

Sharapova's two-set win was also the result needed by Agnieszka Radwanska to advance to the last four in Singapore, having ended the title hopes of top seed Simona Halep earlier on Thursday.

Poland's Radwanska, the fifth seed, had lost both of her opening matches in the round-robin stage but pulled off a 7-6 (7/5) 6-1 victory over world number two Halep, including overturning a 5-1 deficit in the tie-break, to put herself back in the running for a semi-final spot.

Her progression then relied solely on Sharapova's result but the Russian came up with the goods to see them both through to the next round.

"That tie-break was something else and I just managed to make some great points at important moments," Radwanska said after her win.

"It's very nice to hear people say my tennis is entertaining so I hope I can keep hearing people say that over and over again."