DOHA, Qatar — Venus Williams defeated Elena Dementieva 6-4, 4-6, 6-3 Wednesday to win her second round-robin match at the WTA's season-ending Sony Ericsson Championships.

Williams had 10 double-faults to five for Dementieva, but converted eight of 10 break points.

"It was just really competitive out there," Williams said.

Serena Williams defeated Dinara Safina, 6-4, 6-1, and Vera Zvonareva stopped an ailing Ana Ivanovic 6-3, 6-7 (5), 6-4, avenging a lopsided loss to the Serbian in the final last month at Linz, Austria.

The Williams sisters will play each other on Thursday.

Zvonareva won on her third match point when Ivanovic, who took a medical time-out in the first set, hit a forehand into the net.

The French Open champion said she had a sore throat and hadn't eaten much in the past couple of days because of a virus.

"It was very hard for me to breathe," Ivanovic said. "I was struggling."

The ninth-ranked Zvonareva improved to 3-4 against Ivanovic. Zvonareva has won two group matches while Ivanovic has lost two. They have one more round-robin match.

The top eight players are divided into two groups, with the top two in each group advancing to an elimination stage.