Istanbul, Turkey (Sports Network) - Two-time titlist Serena Williams improved to 2-0 in round-robin play at the season-ending WTA Championships in Istanbul.

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

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 world No. 1 Australian Open champion and U.S. Open runner-up Victoria Azarenka, Williams (2-0), Angelique Kerber (0-1) and Li (0-1), while Group B boasts 2004 champion and 2007 WTA Championships runner-up Maria Sharapova (1-0), Wimbledon runner-up Agnieszka Radwanska (1-0), French Open runner-up Sara Errani (0-1) and Sam Stosur, who replaced last year's Championships winner Petra Kvitova in the field after Kvitova (0-1) pulled out, citing a viral illness, after losing to Radwanska on Tuesday.

The top-two players in each group will advance to the semifinals and the winners will square off in the lucrative finale on Sunday.

Azarenka needs only two wins this week to end the year at No. 1.

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

The Sports Network