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 Karolina Pliskova in her final game and can still go through.
Svitolina will top Purple Group if she wins her final match against Bianca Andreescu, or if Pliskova beats Andreescu later on Wednesday.
Svitolina and Halep were well matched for large parts of the gruelling match but Svitolina was more consistent at crucial moments.