By John O'Brien

SINGAPORE (Reuters) - 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 on Tuesday, the world number one virtually booking a spot in the semi-finals of the elite eight-woman event.

In the second Red Group encounter, 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 Sunday, 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."

Kerber came out flying, pushing Halep all around the court with her pinpoint groundstrokes to manufacture an immediate break, which the German consolidated with a gutsy hold after the Romanian set about attacking her opponent's second serve.

Halep was struggling to contain Kerber's main weapon, her whipped forehand, and while the world number four's athleticism and speed around the court kept her in the rallies, the German left-hander continued to dictate the early exchanges.

HIGH QUALITY

The Romanian, however, was showing more patience and opted to keep Kerber pinned on the baseline rather than going for winners, the strategy paying off as she worked her angles beautifully to break back then hold to love for a 4-3 lead.

Kerber was feeling the pressure but maintained her composure and consistency to first stave off her rampant opponent in the eighth game and then pressure Halep into a string of errors to cough up a decisive break of serve in the next.

The German held comfortably to seal a high quality opening set in 43 minutes and carried the momentum into the second when she reeled off her fifth game in a row to open a 2-0 lead before Halep finally stopped the rot by holding.

The Romanian threw everything she had at Kerber in the next game in an attempt to get the set back on serve but the top seed's remarkable retrieving skills and ability to hit winners while stretched out wide neutralized the threat.

Exerting all that effort for no reward seemed to knock the stuffing out of Halep and Kerber pounced again to seize another break against a demoralized opponent to give her the breathing room she needed to seal an ultimately comfortable victory.

In the late match, Keys (1-1) looked a completely different player to the one who surrendered meekly to Halep on Sunday 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.

(Editing by Pritha Sarkar)