The Russian world number four topped the Red Group at the eight women end-of-season finals with her second win and next faces U.S. 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.”
Sharapova edged Radwanska over three tough sets on Sunday but showed no willingness to go the distance again on Tuesday in a ferocious baseline battle against Halep, the top seed in the absence of fatigued world number one Serena Williams.
Both players traded early breaks with neither enjoying an easy ride on serve before five-times grand slam champion Sharapova secured the set by breaking Halep, runner-up here last year, for a second time at 5-4.
The 28-year-old Russian, who had won all five previous matches against Halep, was controlling the bulk of the rallies from the centre of the court with some rangy defensive work also causing the world number two angst.
Sharapova, runner-up to Serena Williams at the Australian Open this year, broke Halep again in a lengthy second game of the second set and some blistering winners from both flanks allowed her to open up a 5-1 lead.
The Russian was two points from victory when her double fault demons returned and Halep seized on the chance to break twice to put the match back on serve.
The rally was short-lived, though, as the shrieking Sharapova broke in the next game to seal the match with a bruising backhand before letting out a roar of approval.
Halep, finalist here last year, faces a weary-looking Radwanska next and was confident of progressing against the Pole, who completed a hectic recent schedule in order to qualify for the lucrative championships.
“I feel okay. I feel ready to play against her,” the Romanian said. “I feel that I played good tennis today, even if I lost. I have confidence in my game.”