Former French Open champion Li Na won a round-robin match Thursday to keep her semifinal hopes alive at the season- ending WTA Championships in Istanbul.

The eighth-seeded Chinese star improved to 1-1 this week with a 6-4, 6-3 handling of fifth-seeded German Angelique Kerber, the German left-hander who went a disappointing 0-3 in her round-robin matches at Sinan Erdem Dome.

Also on Day 3, world No. 1 Victoria Azarenka is battling third-seeded former top-ranked star Serena Williams and seventh-seeded Sara Errani will take on ninth-seeded Samantha Stosur, who replaced 2011 WTA Championships titlist Petra Kvitova in the prestigious eight-woman field on Wednesday.

Williams is seeking a fifth win in five tries against Azarenka this year. The American great beat the Belarusian in last month's U.S. Open finale and is a lopsided 10-1 lifetime against Azarenka.

The 23-year-old Azarenka needs only one more win this week to end the year at No. 1.

The Australian Open champion Azarenka was last year's WTA Championships runner-up to the Czech southpaw Kvitova.

The 2012 Wimbledon, U.S. Open and Olympic champion Williams captured WTA Championships titles in 2001 and 2009 and is a two-time runner-up at this prestigious event.

This week's four-player Red Group features Azarenka (1-0), Williams (2-0), Kerber and Li, while the White Group boasts 2004 champion and 2007 WTA Championships runner-up Maria Sharapova (2-0), Wimbledon runner-up Agnieszka Radwanska (1-1), the French Open runner-up Errani (0-1) and the replacement Stosur.

The top-two players in each group will advance to the semifinals, with the winners squaring off in the lucrative final on Sunday. Sharapova and Williams have already secured berths in the final four.

The winner of this $4.9 million event could collect as much as $1.75 million.