Clijsters, ranked second in the world, won a tough first set in which both players made numerous unforced errors. But the Belgian took control in the second set with aggressive play to extend her winning streak to 23 matches in North American summer hardcourt events.

"When you play against Martina, you have to win the points against her," said Clijsters. "She's not a player who's going to hit you off the court. I know that it's up to me to be aggressive and go for the winners. It's also why you take more risks and make more unforced errors."

Clijsters' opponent in Saturday night's semifinal will be Czech teenager Nicole Vaidisova, a 3-6, 6-4, 6-3 winner over Russia's Anna Chakvetadze.

Second-seeded Maria Sharapova advanced to the other semifinal, routing defending champion Mary Pierce 6-2, 6-3. Sharapova will face fifth-seeded Patty Schnyder, who beat fourth-seeded Elena Dementieva 6-4, 6-3.

Hingis committed 34 unforced errors, while Clijsters had 27. Hingis also hit just 18 winners, compared to 25 for Clijsters.

Clijsters was able to save seven of nine break points she faced. The biggest save came in the sixth game of the first set when Clijsters held a 3-2 lead.

The eight-seeded Hingis was unable to convert on four break points in a game that lasted 15 points before Hingis hit a backhand drop shot wide to put Clijsters ahead 4-2.

Hingis battled back to win the next three games for a 5-4 lead, but Clijsters served out the next game at love to tie the match at 5-all. Clijsters won the final two games to finish off the set.

"You kind of run out of steam when you lose such a set like that," Hingis said.

Clijsters had the only two service breaks in the second set as she completed her third victory over Hingis this year, including wins in the quarterfinals of the Australian and French opens.

"I think I played a good second set," Clijsters said. "But the serve wasn't always where I wanted it to be."

Hingis, who has won five Grand Slam titles, is playing for the first time in three seasons after being sidelined with injuries.

"She puts so much pressure on you all the time," Hingis said. "She doesn't really give you a free point. She's always on top of you."

Sharapova, ranked No. 4, won her third consecutive straight-set match as she overwhelmed Pierce. The Russian teenager ran off seven straight games to overcome a 2-1 first-set deficit and take a 2-0 lead in the second.

"I thought I played solid," said Sharapova, who reached the semis for the eighth time in nine tournaments this season. "After that first break in the second set, I felt pretty much in control."

Sharapova wasted little time dispatching Pierce, who was playing in her third tournament after missing six months with injuries.

"She's always dangerous just because she has all the experience behind her," said Sharapova, the 2004 Wimbledon champ. "Obviously, she has not played a lot, but you really can't worry about that because she can swing freely and hit some big shots."

Pierce, who did not move well throughout the match, played with her right thigh wrapped, but she called it "preventative."

Schnyder continued her recent hot streak, reaching the semifinals for the third straight week. Schnyder advanced to the semis at Cincinnati, and last week lost to eventual champion Clijsters at the Bank of the West Classic in Stanford, California.

Schnyder knows that she has a tough match waiting against Sharapova, who has one title at Indian Wells to her credit this season.

"She is mentally so tough, she can focus on each point and come up with the big shots on the big points," Schnyder said. "It's a tough one. I have to make her run and move and mix it up again. I know I have a chance."

The seventh-seeded Vaidisova made a solid comeback after dropping the first set, then falling behind 2-0 in the second set.

"I don't think it could have gotten much worse than that," Vaidisova said. "I think it would have been even tougher and a little different story if I was down 3-0 and two breaks."

Vaidisova, ranked 12th, will move into the top 10 next week for the first time. Vaidisova, who lost in the French Open semifinals, lost to Clijsters in the semifinals last week.