Istanbul, Turkey (SportsNetwork.com) - World No. 1 superstar Serena Williams stayed perfect for the week, while in a battle of former world No. 1s, seventh-seeded Jelena Jankovic upset second seed Victoria Azarenka in round- robin action Wednesday at the season-ending WTA Championships in Istanbul.
The reigning champion Williams improved to a brilliant 75-4 this year by topping third-seeded Agnieszka Radwanska 6-2, 6-4 in a rematch of last year's Wimbledon final, which was also won by the powerful American.
This week's four-woman Red Group is led by the U.S. and French Open champion Williams (2-0) and also includes Radwanska (0-2), fifth-seeded 2011 titlist Petra Kvitova (1-0) and eighth seed Angelique Kerber (0-1).
The three-time Championships titlist and two-time runner-up Williams beat Maria Sharapova in last year's finale in Istanbul.
The Serbian star Jankovic, meanwhile, straight-setted her fellow U.S. Open runner-up Azarenka in 6-4, 6-3 fashion on the indoor hardcourt at Sinan Erdem Dome.
The 2011 WTA Championships runner-up Azarenka (1-1) spearheads this week's four-player White Group, which also features fourth-seeded Aussie Open runner-up Li Na (1-0), sixth-seeded former French Open finalist Sara Errani (0-2), and Jankovic (1-0).
Also in the White Group on Day 2, Li took care of an all-but-eliminated Errani, 6-3, 7-6 (7-5).
On Thursday, Williams will square off against the 2011 Wimbledon champ Kvitova, Radwanska will meet Kerber, and Li will battle the resurgent Jankovic.
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 through Friday, followed by semifinal action on Saturday, and a lucrative final on Sunday.