Serena Williams survived an injury scare here Monday to make a winning start in her bid to cap a brilliant comeback season by regaining the WTA Championship title in the tour's season-end event.

The Wimbledon, US Open and Olympic champion appeared to pull a gluteal muscle in the first set while attempting a vigorous retrieve, but played through the discomfort to overcome Angelique Kerber, a debutant in this top eight event.

Williams' tenacious efforts eventually earned her a 6-4, 6-1 revenge over the improving German left-hander, who upset the American tennis legend in their last meeting, at Cincinnati two months ago, and who had her moments early on this time too.Kerber was surprisingly aggressive while coming back from 0-3 to 3-3 in the first set, and also played some good rallies while earning three chances to reach 5-5.

But she could not convert any of them, and once Williams broke for 3-1 in the second set with a trademark ferocious drive volley, the match quickly slid away from the woman from Kiel.

There had nevertheless been a few moments in the fifth and sixth games when a different outcome seemed possible, with Williams labouring to reach the ball and apparently anxious about her restricted movement.

Once she held her lower back after failing to reach a ball wide on the forehand side and she dropped serve in that game after unaccountably hitting wide with a backhand drive from well inside the baseline.

As Kerber began to dictate some of the rallies in the following game, Williams again appeared to hold her gluteal region. But her rhythmic service action helped get her through the next few minutes and after she held serve again for 5-4 Williams pumped up her adrenaline with fearsome fist pumping and yelling.

Her standard immediately improved, but Kerber might still have reached 5-5 had she not double faulted on an advantage point. That failing was punished immediately by two solid Williams attacks which snatched her the set.

At the end Williams' celebrations seemed subdued, and it was an hour before she appeared for a press conference. She has little time in which to recover from any after-effects for on Wednesday she is scheduled first on against Li Na, the former French Open champion from China.

Williams denied that she had suffered physical problems, saying: "I feel like I am moving well, getting a lot of balls back,."

She suggested instead that she had taken a while to get into her rhythm, not having competed since winning the US Open more than six weeks ago.

"I felt like I was ready to play," she claimed. "I felt I had practised too much, and if I hit another practice ball I would go nuts."

Kerber must now create a surprise if she is to retain any hopes of reaching the semi-finals, as her next group match is against the world number one, Victoria Azarenka, also tomorrow.

Earlier the chances of the title changing hands increased signficantly when Petra Kvitova, the defending champion, produced a downbeat and error-prone performance to lose 6-3, 6-2 to Agnieszka Radwanska, the Wimbledon finalist.

It was the first time in four meetings that the Czech had lost to the Pole.

Kvitova had been below par while losing in the first round in Tokyo at the end of last month, and now again looked far from her fittest, spraying a shower of 41 unforced errors compared with a mere five from Radwanska.

Only briefly did the champion appear capable of gaining control, while she was recovering from the early loss of a service game and a 0-3 deficit to level at 3-3.

Thereafter she seemed to lack belief as well as consistency.

Later Kvitova appeared tearful and upset both by her performance and the result, admitting that her sadness and anger had increased after returning to the locker room.

"I was not very comfortable on court and I didn't feel pretty well," said the woman who had basked in some of her happiest moments here last year.