Angelique Kerber beats Dominika Cibulkova as Simona Halep beats Madison Keys in WTA finals

World number one Kerber had to battle in her opening match with the Slovakian Cibulkova forcing a tie-break in the opening set which Kerber won 7-5.

Cibulkova responded well winning the next set 6-2. Kerber battled back to win the final set 6-3 and secure a first win of the finals.

"Domi is a tough opponent - I knew that I had to play my best tennis," said Kerber after the game.

"I was trying to stay positive in the third set, play my game, be aggressive and just go for it. I was also really concentrating on getting my first serve in. It was a really close match."

Earlier on Halep proved that she will be a formidable opponent at the elite eight-woman event at the Singapore Indoor Stadium.

"It's always the same with Madison... she's very strong and hits the ball hard so it's tough to match her power," said Halep. "So I have to adjust, be strong with my legs and make the most of my chances."

Both players got off to solid starts and held comfortably with Keys making full use of her booming forehand to keep Halep at bay.

It was the diminutive Romanian who drew first blood in the third game, however, probing down the American's backhand side with pinpoint accuracy and converting her fourth break point when Keys plonked a weak drive into the net.

Buoyed by the early breakthrough, Halep went for the kill and rattled off the next 11 points on her way to a 5-1 lead as Keys' game unravelled and became riddled with unforced errors.

The 21-year-old American stopped the rot with a comfortable hold when Halep, pushing a little too hard, sent a few returns long but the Romanian's body serves paid dividends in the next game as she wrapped up the first set in just 27 minutes.

Halep swarmed all over Keys at the start of the second set, looking busy and hustling around the court to earn two more break points before the American rediscovered her range with the forehand to stave off the threat and claim a vital hold.

Keys, however, continued to mix winners with errors until her luck ran out in the fifth game, when another forehand into the net handed Halep the first break of the second set, which the Romanian consolidated with a hold to love.

Seemingly out of nowhere, Keys was able to capitalise on her opponent's lapse in concentration to level the set at 4-4 but a fired up Halep broke back immediately before serving out the contest in the next game.

Defending champion Agnieszka Radwanska opens her campaign against Russia's Svetlana Kuznetsova on Monday with Garbine Muguruza and Karolina Pliskova completing the White Group.