Serena rolls; Jankovic upsets Azarenka in Istanbul

Istanbul, Turkey ( - World No. 1 superstar Serena Williams stayed perfect for the week, while in a battle of former top-ranked stars, 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 converted on her third match point to take care of an all-but-eliminated Errani, 6-3, 7-6 (7-5). The Chinese star is now 6-0 lifetime against the gritty Italian.

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.