Li, Kvitova reach Championships semis

Istanbul, Turkey ( - 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.

Li was joined in the semis by fifth-seeded former Wimbledon runner-up Petra Kvitova, who snuck past eighth seed Angelique Kerber 6-7 (3-7), 6-2, 6-3.

Friday's round-robin action opened with the fourth-seeded Li leveling a hobbled Azarenka 6-2, 6-1 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 featured sixth-seeded Sara Errani (1-2) and seventh seed Jelena Jankovic (1-2), who had already secured a spot in Saturday's final four before losing to Errani in Friday's last round-robin affair, as Errani doused the former No. 1 and former U.S. Open runner-up 6-4, 6-4.

The four-woman Red Group was captured by Serena Williams (3-0). That group also featured third seed Agnieszka Radwanska (0-3), the 2011 Championships champion Kvitova (2-1) and Kerber (1-2). The U.S. and French Open champion Williams easily rolled into the semis, where she was joined from the Red Group by Kvitova, who will lock horns with Li here on Saturday.

Williams will take on a resurgent Jankovic on Saturday in a rematch of the 2008 U.S. Open final, which was won by the formidable American. Williams is 7-4 lifetime versus Jankovic, including 2-0 this year in a pair of finals in Charleston and Beijing.

Li and Kvitova will meet for a seventh time, with the two women splitting their first six matchups. The Czech left-hander topped the Chinese veteran in Beijing three weeks ago.

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.

An undefeated champion this week, which could only be Williams or Li at this point, would stand to earn $2.1 million.