Serena Williams survived an injury scare on 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.
Kerber faces another stern challenge in the competition meeting world number one Victoria Azarenka, who begins her campaign on Wednesday.