Angelique Kerber beat Naomi Osaka at the WTA Finals to leave the US Open champion on the brink of elimination at the group stage.
Germany's Kerber, 30, has one win from two in the group, while Osaka, 21, has lost both of her round-robin matches.
Kiki Bertens and Sloane Stephens, who both won their opening matches in Singapore, play later on Wednesday.
Kerber, the number one seed, beat the Japanese 6-4 5-7 6-4 in a match that included 11 breaks of serve.
The first hold of the match came in the fourth game from Kerber.