World No. 1 Victoria Azarenka saved two match points in her opening-match victory, while former champion Maria Sharapova and two-time titlist Serena Williams each improved to 2-0 in round-robin play at the season-ending WTA Championships in Istanbul.

Azarenka staved off fifth-seeded German left-hander Angelique Kerber 6-7 (11-13), 7-6 (7-2), 6-4 in a high-quality 3-hour, 6-minute battle on Wednesday. The Belarusian star saved match points en route to forcing a second-set tiebreak, which she wound up dominating before coming out on top in a tight third.

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

Azarenka and Williams will square off on Thursday, with the American seeking a fifth win in five tries this year. Williams beat Azarenka in last month's U.S. Open finale and is a lopsided 10-1 lifetime against the Belarusian.

The Australian Open champion Azarenka was last year's WTA Championships runner-up to Czech southpaw Petra Kvitova, who pulled out of the Istanbul field Wednesday, citing a viral illness.

The second-seeded former No. 1 Sharapova came from behind to best fourth- seeded Wimbledon runner-up Agnieszka Radwanska 5-7, 7-5, 7-5 in 3 hours, 12 minutes in Wednesday's nightcap, which went past 2 a.m. local time. The tall Russian star broke for a 6-5 edge in the final set and closed out the match with a final service hold.

Meanwhile, the third-seeded former top-ranked great Williams overcame a shaky service game and got past eighth-seeded former French Open champion Li Na, 7-6 (7-2), 6-3, indoors at Sinan Erdem Dome. The powerful American smacked six aces in the 1-hour, 50-minute victory.

Williams uncharacteristically dropped her serve five times on Wednesday and got so frustrated at one point she smashed her racquet on the court.

"No, I don't break racquets in matches. Although I didn't hit it hard," she said. "Maybe I don't know my own strength."

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 Group A features Azarenka (1-0), Williams (2-0), Kerber (0-2) and Li (0-1), while Group B boasts the 2004 champion and 2007 WTA Championships runner-up Sharapova (2-0), Radwanska (1-1), seventh-seeded French Open runner-up Sara Errani (0-1) and Sam Stosur, who replaced last year's Championships winner Kvitova in the field.

The top-two players in each group will advance to the semifinals, with the winners squaring off in the lucrative final on Sunday.

Also in Istanbul on Thursday, Kerber will face Li and Errani will tangle with a ninth-seeded Stosur.

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