Istanbul, Turkey (SportsNetwork.com) - Chinese star Li Na prevented world No. 2 Victoria Azarenka from reaching this weekend's semifinals at the season- ending $6.5 million WTA Championships.

Friday's round-robin action opened with the fourth-seeded Li leveling a hobbled Azarenka 6-2, 6-0 on the indoor hardcourt at Sinan Erdem Dome. It marked a rematch of January's Australian Open final, which was won by the Belarusian star.

Azarenka injured her back on Day 4 and required treatment during several changeovers amid the uneventful setback versus Li.

The former French Open champion Li went a perfect 3-0 in this week's four- player White Group, while the former world No. 1 and 2011 Championships runner-up Azarenka went 1-2, including losses in her last two matches in Istanbul.

The White Group also features sixth-seeded Sara Errani (0-2) and seventh seed Jelena Jankovic (1-1), who has already secured a spot in Saturday's final four. The former No. 1 and former U.S. Open runner-up Jankovic will meet Errani later Friday in a meaningless affair in the standings.

The four-woman Red Group was captured by Serena Williams (3-0). That group also features third seed Agnieszka Radwanska (0-3), fifth-seeded 2011 champion Petra Kvitova (1-1) and eighth seed Angelique Kerber (1-1). The U.S. and French Open champion Williams will play in the semis and will be joined there from the Red Group by Friday's Kvitova-Kerber winner. Li awaits the Kvitova- Kerber victor.

Williams will take on Jankovic on Saturday in a rematch of the 2008 U.S. Open final, which was won by the powerful American.

The three-time champion and two-time runner-up Williams is the reigning WTA Championships titlist, having beaten Maria Sharapova in last year's finale in Istanbul, which is hosting this elite eight-woman event for a third and final time before it shifts to Singapore next year.

The 32-year-old Williams, who's a stellar 76-4 this season, headed to Istanbul having already clinched the year-end No. 1 spot for the third time in her brilliant career.

Round-robin play is being staged through Friday, followed by semifinal action on Saturday, and the lucrative final on Sunday. An undefeated champion this week, which could only be Williams or Li at this point, would stand to earn $2.1 million.