Istanbul, Turkey (Sports Network) - Former champion Maria Sharapova and two- time titlist Serena Williams were easy round-robin winners, as the 2012 WTA Championships commenced Tuesday in Istanbul.

The second-seeded former world No. 1 Sharapova leveled seventh-seeded Sara Errani 6-3, 6-2, while the third-seeded former top-ranked great Williams took care of fifth-seeded German left-hander Angelique Kerber 6-4, 6-1 indoors at Sinan Erdem Arena. The Russian Sharapova also beat the Italian Errani in this year's French Open final.

The 2012 Wimbledon, U.S. Open and Olympic champion Williams captured WTA Championships titles in 2001 and 2009 and is a two-time runner-up at this season-ending event.

Play got underway on Day 1 when Poland's Agnieszka Radwanska beat reigning champion Petra Kvitova of the Czech Republic. The fourth-seeded 2012 Wimbledon runner-up to Williams Radwanska whipped the sixth-seeded 2011 Wimbledon champion Kvitova, 6-3, 6-2.

Kvitova defeated current world No. 1 Victoria Azarenka in last year's final in Istanbul, which will also play host to this elite eight-player tournament next year.

This week's four-player Group A features the current No. 1 and Australian Open champion Azarenka, Williams (1-0), Kerber (0-1) and eighth-seeded Li Na, while Group B boasts the 2004 champion and 2007 WTA Championships runner-up Sharapova (1-0), Radwanska (1-0), Kvitova (0-1) and Errani (0-1).

The top-two players in each group will advance to the semifinals, and the winners will square off in the lucrative finale on Sunday.

Azarenka needs only two wins this week to end the year at No. 1.

The winner of this $4.9 million event could collect as much as $1.75 million.

The Sports Network