Venus endured a struggle against Elena Dementieva 6-4, 4-6, 6-3 to win her second match, and Serena romped past Dinara Safina 6-4, 6-1 in her first at the season-ending WTA Tour championships.
On Thursday, they meet for the 18th time in their professional careers, with Serena holding a 9-8 edge.
"Tomorrow I'll definitely be going for the win, and just taking everything that I can,'' said seventh-seeded Venus, who would qualify for the semifinals with a win.
"It's going to be good and exciting,'' third-seeded Serena said. "I think she's pretty much already in the semis. And I've got to get there or I want to get there. So I know it's not going to be easy.''
Serena barely got the better of her older sister in their last contest, the U.S. Open quarterfinals, and went on to win the title, her ninth major. She lost to Venus in the Wimbledon final, and the pair teamed up for Olympic gold in the doubles at Beijing.
Venus rallied from 2-0 down in the last set against Dementieva. She had 10 double-faults to five for the Russian but converted eight of 10 break points. The two engaged in powerful baseline rallies.
"It was just really competitive out there,'' the older Williams said.
The points were much shorter between Serena Williams and second-seeded Safina, whose second loss eliminated her from semifinals contention.
Vera Zvonareva remained unbeaten in handing an ailing Ana Ivanovic her second defeat, 6-3, 6-7 (5), 6-4.
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, who beat Zvonareva in the Linz final last month, 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.
The top eight players are divided into two groups, with the top two in each group advancing.
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