Clijsters easily bounced fourth-seeded Justine Henin, 6-2, 6-1, in a battle if Belgians Friday afternoon.
Willams improved to 9-1 in her career against Seles, overcoming 38 unforced errors with a powerful serve that produced 11 aces. Williams also saved 10 of 13 break points while Seles had 22 unforced errors and just one ace.
Williams also had a simple answer for what made the difference, saying, "I didn't want to lose."
In the 10 meetings, Williams has dropped just four sets to her American counterpart. Two of the victories came this year in majors at the French Open and U.S. Open.
Despite her dominance in the matchups, Williams still considers Seles a dangerous player.
Henin lost for the third time in four meetings this year with her compatriot and friend, falling to 2-5 overall. This was the most lopsided result.
"I think I was consistent throughout the whole match," said Clijsters, who prefers indoor tournaments. "I didn't have a period where I started to make a little more errors. I think that was the main key. I feel more comfortable playing indoors."
"Maybe that's the worst I ever played against her," Henin admitted. "But she played solid, for sure. She played a good match and she played well in the last tournaments in the last three months. She played much better than me, and she's in the semifinals."
Henin also lost to Clijsters at Sydney and the Australian Open. Her lone 2002 win against Clijsters came on clay at Rome.
Despite the loss, it was a terrific year for Henin, who won two titles and lost to the powerful Williams sisters in four finals as she climbed to a career-high fourth in the rankings.
"I had a great year, for sure," said Henin, who is getting
married next week. "I didn't expect to finish the end in the top five. That is good for me and unbelievable for me. It's a little bit hard to finish like this, but (overall) it's pretty positive."
Clijsters won three titles and was a finalist in two more events. Despite her dominance of Henin, there are no hard feelings as she will be in Henin's wedding party.
"I always did pretty well at the championships at the end of the year," said Clijsters, who was a semifinalist here last year. "I think I'm motivated to try to finish the year on a good note."
"She's definitely not almost finished," Williams said of Seles. "Tonight she showed everyone that she's very much a force to be reckoned with and extremely serious about her game."
By beating Seles again, Williams moved within two wins of claiming her eighth title of the year. She also was a finalist at French Open, Wimbledon and the U.S. Open, falling to her younger sister Serena each time.
"She comes up with the goods and puts pressure on you with her serve and movement," Seles said. "I think that's why she is No. 2 and I'm whatever."
Saturday, the other semifinal will be decided when top-seeded Serena Williams faces fourth seed Jelena Dokic and No. 3 Jennifer Capriati meets unseeded Magdalena Maleeva.
This event offers a total purse of $6 million, with the winner taking home $765,000.