World number one Angelique Kerber and Slovakia's seventh seed Dominika Cibulkova secured semi-final places at the WTA Finals as both players won their final group matches in Singapore.
Kerber beat American Madison Keys 6-3 6-3 to claim her third win of the week.
Cibulkova had required the German to win in straight sets to progress from the White Group, having earlier beaten Romania's Simona Halep 6-3 7-6 (7-5).
Russia's Svetlana Kuznetsova had already qualified from the Red Group.
The winner of Friday's final round-robin match between Poland's defending champion Agnieszka Radwanska and Czech fourth seed Karolina Pliskova will complete the semi-final line-up.