Istanbul, Turkey: Poland's Agnieszka Radwanska defeated reigning champion Petra Kvitova of the Czech Republic, 6-3, 6-2 as the 2012 WTA Championships kicked off here.
In another White Group match Tuesday, former world No. 1 Maria Sharapova of Russia, seeded number two, beat seventh-seed Sara Errani 6-3, 6-2. The Russian Sharapova also beat the Italian in this year's French Open final, reports Xinhua.
Third-seed Serena Williams took care of fifth-seeded German Angelique Kerber 6-4, 6-1 in the Red Group.
The single round robin will continue to Friday and the top two players in each group will advance to the semifinals, and the winners will square off in the final on Sunday.