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. Defending champion Agnieszka Radwanska, Svetlana Kuznetsova, Karolina Pliskova and Garbine Muguruza are in the other.
Kerber ended Serena Williams' 186-week stay at world number one in the wake of winning the US Open in September and her trademark precise forehand, which also helped her to the Australian Open title in January, proved too potent as Halep was forced out wide and out of contention.
Kerber played the big points especially well, converting four of five break points, compared with Halep's return of just one break from six opportunities.
She has won only twice on her three previous appearances at the season finale and has never been beyond the group stages.
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