Istanbul, Turkey (SportsNetwork.com) - Reigning champion Serena Williams and former world No. 1 star Victoria Azarenka were a pair of opening-day winners at the season-ending WTA Championships in Istanbul.

The top-seeded Williams, who will finish as the year-end No. 1 for the third time in her career, waltzed past eighth-seeded German left-hander Angelique Kerber 6-3, 6-1 in 64 minutes on the indoor hardcourt at Sinan Erdem Dome on Tuesday.

This week's four-woman Red Group is led by the U.S. and French Open champion Williams (1-0) and also includes third-seeded Agnieszka Radwanska (0-1), fifth-seeded 2011 titlist Petra Kvitova (1-0) and Kerber (0-1).

The three-time Championships titlist and two-time runner-up Williams beat Maria Sharapova in last year's finale in Istanbul.

Meanwhile, the second-seeded Azarenka got past sixth-seeded Italian Sara Errani 7-6 (7-4), 6-2. The Australian Open champion Azarenka was down a pair of breaks, trailing 2-5 in he first set, before charging back to overtake the former French Open runner-up Errani. The marathon first set took 1 hour, 27 minutes to complete.

"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.

The 2011 Championships runner-up Azarenka (1-0) spearheads this week's four- player White Group, which also features Errani (0-1), fourth-seeded Aussie Open runner-up Li Na and seventh-seeded Jelena Jankovic.

In the opening-day nightcap, the former Wimbledon champion Kvitova looked impressive in upending the former Wimbledon runner-up Radwanska 6-4, 6-4.

Round-robin play will resume Wednesday, as Williams meets Radwanska, Azarenka takes on her fellow U.S. Open runner-up Jankovic and the former French Open champion Li faces Errani, who is nursing a calf injury.

Istanbul is hosting this elite eight-woman event for a third and final time before it shifts to Singapore next year.

Round-robin play will be staged Tuesday through Friday, followed by semifinal action on Saturday, and a lucrative final on Sunday.