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Maria Sharapova continued her mastery of Romania's Simona Halep with a high quality 6-4 6-4 victory filled with bruising baseline rallies to edge closer to a last-four berth at the WTA Finals.

The Russian world number four topped the Red Group at the eight women end-of-season finals with her second win and next faces US Open champion Flavia Pennetta, who said she would retire after the tournament, with a semi-final spot on the line.

Pennetta had earlier kept alive her WTA Finals bid with a flurry of backhand winners as she downed a strapped up and fatigued Agnieszka Radwanska 7-6(5) 6-4 to leave the Pole bottom of the standings with two defeats.

It was Pennetta's first win in the group - she was blown away by top seed Halep 6-0 6-3 in her opener on Sunday - with her backhand proving a particularly devastating weapon at the Singapore Indoor Stadium.

Sharapova's win over the Romanian means all four players in the group can still claim one of the two semi-final spots available but the Russian's strong form on her return after injury problems since Wimbledon has put her in the driving seat.

"I'm quite pleased to be able to beat the number two player in the world," she told reporters.

"I don't think that I played unbelievable tennis today. I still made errors, but cut down on my errors from the first match."

Source: Reuters