Angelique Kerber produced a polished display to claim her second straight win at the WTA Finals with a 6-4 6-2 victory over Simona Halep, the world No.1 virtually booking a spot in the semi-finals of the elite eight-woman event.

In the second Red Group encounter on Wednesday, Madison Keys claimed her first win of the tournament in a battle between debutants when the American used her powerful forehand to overpower Slovakia's Dominika Cibulkova 6-1 6-4.

Kerber (2-0), who beat Cibulkova in her first match on Monday, was at her resilient best against Halep (1-1), standing firm when her opponent threatened and putting her foot on the accelerator when she sniffed an opportunity to pull away.

The German entered the match trailing Halep, who thrashed Keys in her Red Group opener, 4-3 in their head-to-head meetings but the 28-year-old showed flashes of the form that brought her two grand slam titles in 2016.

"I'm feeling very, very good here and it's just great to win my second match here playing my best tennis," Kerber said.

"I'm trying to enjoy all my matches here and I hope the spectators can enjoy it as well."

In the late match, Keys (1-1) looked a completely different player to the one who surrendered meekly to Halep on Monday when she jumped all over Cibulkova's (0-2) serve to race through the first set in just 23 minutes.

The Slovak improved to make a contest of it in the second set but a single break in the fifth game was all Keys needed to seal the contest, although the permutations available in the final pool games means all four players can still advance.