SAN JOSE, Calif. - Andre Agassi struggled early against Vladimir Voltchkov on Friday night. Then he quickly found his touch. The top-seeded Agassi saved a match point in the second set and cruised through the third en route to a 6-7 (3), 7-6 (6), 6-1 win in the quarterfinals of the Siebel Open.

"I was well aware he was playing well and I wanted to make him earn it to the very end," said Agassi, who won his eighth straight match of the season. "There's not a whole lot you can do when a guy is hitting the ball that well."

Voltchkov hit more winners, 49-38, and more aces, 20-13, but also had 56 unforced errors to Agassi's 23.

After playing the first two sets in a little over two hours, Agassi took care of matters in 24 minutes in the third set.

"As far as I was concerned I was nestling in for another hour," Agassi said. "There were no signs of that match getting any easier, but then it opened up."

The unseeded Voltchkov, who reached the semifinals of Wimbledon in 2000, broke Agassi's serve early in the first set. Through seven games, Agassi got just 45 percent of his first serves in and he was 1-of-9 on break points.

"He hits the ball so well off the ground, he immediately takes control of the point," Agassi said. "I've never faced a guy who hits it that well when he's on a dead run. That was a quality match."

Voltchkov won the first set when Agassi double faulted in the tiebreaker.

In the second set, both players held serve, but Voltchkov went up 6-5 in the tiebreaker before Agassi recovered and won 8-6 with a two-handed backhand down the line.

"I make a living on that shot," Agassi said. "I knew I had it."

Agassi broke Voltchkov's serve three times in the final set to reach the semifinals against either No. 3 seed James Blake or No. 7 seed Nikolay Davydenko, who played a later match.

"It all came down to endurance and Andre was stronger than me," Voltchkov said. "He played all three sets at a very high level. He played so fast and so hard it was tough to match him on the court."

Agassi, who lost his serve for just the second time in seven matches, improved to 42-6 lifetime in this event. He will be trying for his eighth final and fifth title.

Justin Gimelstob also advanced with a 6-3, 6-4 victory over Denmark's Kenneth Carlsen in another quarterfinal.

Gimelstob was happy because he reached his first ATP semifinal in nearly three years, and just his fourth since turning pro in 1996.

"This is my first semifinal in a long time and that last game was tough," Gimelstob said. "I was feeling a little nervous and I wanted to close it out. It was relief, really."

In another quarterfinal match, Italy's Davide Sanguinetti beat Vincent Spadea 6-1, 7-5 and will face Gimelstob in one of Saturday's semifinals.

Sanguinetti and Gimelstob were both All-Americans at UCLA a few years apart.

Gimelstob's victory over the eighth-seeded Carlsen was his second over a seeded player. He upset No. 2 seed Paradorn Srichaphan in the first round.