Holland's Kiki Bertens qualified for the semi-finals of the WTA Finals after opponent Naomi Osaka retired from their group match in Singapore.
Osaka took a medical timeout with what appeared to be a leg injury at 5-3 down in the opening set and, after Bertens won the next game to take the set, the US Open champion indicated she was unable to continue.
The winner of Friday's other match in the Red Group between Angelique Kerber and Sloane Stephens will join Bertens in the semi-finals.
Stephens edges out Kerber
Former US Open champion Stephens came out on top in a hard-fought contest with Kerber, winning 6-3 6-3 in an hour and 40 minutes.
Stephens had to save break points in each of her first two service games before breaking Kerber twice to surge into a 5-2 lead, although the 25-year-old then missed an easy forehand on her first set point.
That error threatened to prove costly when Kerber broke serve to keep the set alive, but the left-hander promptly lost her own serve for a third time to gift Stephens the set.
Two more breaks of serve helped Stephens secure her third victory of the week and set up a semi-final with Karolina Pliskova. Bertens will take on Elina Svitolina in the other semi-final.
