Serena Williams of the US hits a return to Angelique Kerber of Germany during their WTA tennis championships match at Sinan Erdem Dome in Istanbul, October 22, 2013. [Photo/Agencies[

ISTANBUL - World number one Serena Williams showed her determination to defend her WTA Championships title by sweeping to a 6-3 6-1 win over Angelique Kerber at the season-ending event on Tuesday.

The 32-year-old American broke early in each set and was too strong for the German, hitting 31 winners.

Kerber, who qualified for the eight-strong tournament only because Maria Sharapova pulled out, won just five games when she was beaten by Williams in her opening match last year.

"Angelique is a great player and right from the start I knew I had to play really well to be able to win this match," Williams said. "I feel really good."

Williams has been drawn with Agnieszka Radwanska, Petra Kvitova and Kerber in the red group at Istanbul's Sinan Erdem dome. Second seed Victoria Azarenka, Li Na, Sara Errani and Jelena Jankovic are in the white group.

Belarussian Azarenka beat Errani 7-6 6-2 in the tournament's opening match with the defensively-minded Italian, who led 5-2 in the first set, staying in contention until the tiebreak.

Errani lost that 7-4 and looked to be in pain after suffering a calf injury midway through the second set.

"I was a little rusty at the beginning," Azarenka told reporters. "Overall it was important to take that first set. It was a big turnaround point."