By John O'Brien

SINGAPORE (Reuters) - Angelique Kerber eased into the last four of the WTA Finals with an unbeaten record when the world number one thrashed Madison Keys 6-3 6-3 in the Red Group finale at the Singapore Indoor Stadium on Thursday.

The 28-year-old German won all three of her round robin matches and her straight sets triumph over Keys ensured Dominika Cibulkova would join her in the semi-finals after the Slovak had earlier ground out a 6-3 7-6(5) victory over Simona Halep.

Kerber has enjoyed a breakout season with grand slam victories at the Australian and U.S. Opens, and her baseline hustling and resilient defence proved far too strong for an erratic Keys, who was broken six times in nine service games.

"It's always tough to play against Madison so I was focused on playing my best and going for it when I had the chance," Kerber said. "I moved and returned well and it feels incredible to be in the semi-finals for a first time."

The first set featured five consecutive service breaks before the German took control by neutralising Keys's powerful forehand.

Kerber staved off another break point in eighth game to edge 5-3 ahead and the German claimed the first set when Keys was unable to hold serve for a fourth straight time.

The American's high-risk aggressive game is vulnerable against a defensive master like Kerber but after hitting too many unforced errors in the first set, Keys found her range at the start of the second to open a 2-0 lead with an early break.

DETERMINED CIBULKOVA

Her revival was short-lived, however, as the tenacious Kerber stepped up another level to reel off five straight games and set up an ultimately routine victory.

Kerber will face either defending champion Agnieszka Radwanska or Karolina Pliskova, a rematch of last month's U.S. Open final, in Saturday's semi-finals while Cibulkova is set for a showdown against White Group winner Svetlana Kuznetsova.

After losing round robin matches to Kerber and Keys, the Slovak needed a straight-sets win to have any chance of advancing. She stayed alive with a determined display before the German world number one completed a perfect day for Cibulkova.

In the early match, the busy Cibulkova came out all guns blazing, looking far more composed than her opponent, harrying Halep on every point and breaking to love at the first opportunity as she raced to a 3-0 lead in less than 10 minutes.

Halep, whose left knee was heavily taped, was struggling to get a foothold in the contest and Cibulkova rode the confidence of a 13-point streak to build a 5-2 lead before sealing the opening set with a big serve that her opponent returned long.

The world number eight started the second set as she had the first but Halep stood firm and recovered an early break to prevent her opponent from running away with the match.

Cibulkova, however, was still on the front foot and calling the shots but after breaking her opponent twice in a row towards the end of the set, the hobbling Halep came storming back but was unable to contain the hyperactive Slovak in the tiebreak.

(Editing by Nick Mulvenney and Pritha Sarkar)