ISTANBUL (AP) -- Top-ranked Serena Williams breezed past Angelique Kerber 6-3, 6-1 to open her bid for a fourth WTA Championships title on Tuesday.
Williams, the defending champion, had no trouble against Kerber, who was one of the few players to beat her in 2012.
At age 32, Williams is enjoying the finest season in her career, having already won 10 titles. She added the French Open and U.S. Open titles to bring her Grand Slam record to 17 championships.
Earlier, second-ranked Victoria Azarenka overcame an erratic performance to beat an ailing Sara Errani 7-6 (4), 6-2 in the opening match of tournament.
''I was a little rusty at the beginning. I think it's a little bit expected after a break, and playing right away against a top player always makes it a little bit difficult,'' Azarenka said.
Errani hurt her right calf in the third game of the second set and took a medical timeout to have it treated. Several times after that she seemed to be bothered by the injury and to favor her right leg, although she returned after the medical timeout to break Azarenka's serve to level at 2-2. She did not win another game though.
Errani served for the set three times - and dropped serve each time as both players produced sloppy play with little rhythm.
Azarenka managed to hold her serve only once through the first eight games but seized her chance when it came.
She won four straight points in the tiebreaker to take a 5-3 lead, then faltered when she served her sixth double-fault. But an error by Errani gave Azarenka two set points. Errani then committed a double-fault and Azarenka had the set.
''I think, overall was important to take that first set. It was a big turnaround point,'' Azarenka said.
Azerenka, who was runner-up in the event in 2011 and a semifinalist last year, finished with eight double-faults but she also had 51 winners for Errani's 28.
Azarenka, the Australian Open champion, lost both of her opening matches in Tokyo and Beijing, where she had 15 double-faults.
The tournament brings together the top eight players in the world. Third-ranked Maria Sharapova dropped out with a shoulder injury.