World number one Angelique Kerber beat Romania's Simona Halep 6-4 6-2 to all but book her place in the last four of the WTA Finals in Singapore.
The German, who beat Dominika Cibulkova in Monday's opener, swept past world number four Halep in 82 minutes.
"I'm feeling very, very good here," said Kerber.
American Madison Keys recovered from losing to Halep in her opening match to see off Cibulkova 6-1 6-4 and stay in the hunt for the semi-finals.
Two players will progress from each of the tournament's two groups to contest the semi-finals.