Defending champion Elina Svitolina beat Simona Halep to become the first player to secure a place in the semi-finals of the 2019 WTA Finals in China.
The Ukrainian world number eight won 7-5 6-3 in Shenzhen to guarantee progression with one group game left.
Halep plays Czech Karolina Pliskova in her final game with the winner qualifying for the semis.
Pliskova beat Bianca Andreescu when the Canadian retired with a knee injury after losing the first set 6-3.
US Open champion Andreescu, who is now unable to qualify for the next round, was leading 2-0 when she landed awkwardly, jarring her left knee and retired despite attempting to continue with heavy strapping.
Svitolina will top Purple Group if she wins her final match against Andreescu, if the 19-year-old is fit to play on Friday.
Svitolina and Halep were well matched for large parts of the gruelling match but Svitolina was more consistent at crucial moments.
After a tight first set, she broke serve to love at 6-5 to take a 1-0 lead, helped by errors off the racquet of her Romanian opponent.
World number five Halep broke early in the second set, but Svitolina hit back immediately and then broke to love again to move 5-3 in front as Halep faltered.
Svitolina had to save two break points when as she served for the match – but eventually clinched the win on her first match point.