World number four Maria Sharapova held her nerve in a tense finish to beat top seed Simona Halep and close in on a last-four place at the WTA Finals.
The Russian withstood a Halep fightback to win 6-4 6-4 and move top of the Red Group in Singapore.
US Open champion Flavia Pennetta earlier beat Agnieszka Radwanska 7-6 (7-5) 6-4 to keep her hopes alive.
Pennetta and Sharapova will meet in the final round of Red Group matches on Thursday, with Halep facing Radwanska.
All four players remain in with a chance of securing one of two semi-final places.