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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 was nursing a sore back on Day 4 and required treatment during the uneventful setback versus Li.

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 world 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.

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.

Round-robin play is being staged through Friday, followed by semifinal action on Saturday, and the lucrative final on Sunday.

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