Andre Agassi [Images] rallied from a set down to edge out Spain's Alex Calatrava 3-6, 6-4, 7-5 and advance to the quarterfinals of the U.S. Men's Clay Court Championships in Houston on Thursday.

The second seeded Agassi was forced to draw on his vast experience when his opponent recovered from a break down in the deciding set to draw level at 4-4.

The American held in the ninth game and wasted a match point in the next before finally securing the victory with a backhand winner down the line two games later.

"I threw up a couple of double faults at a bad time, leading 4-2," Agassi said. "I got too aggressive."

The 34-year-old admitted that he had lost focus at a vital time.

"You don't finish off matches by trying to blast through somebody on clay. You've got to finish off by how you got there which is by discipline, shot selection and work ethic.

"I just lost concentration there."

Agassi next faces Sebastien Grosjean for a place in the last four after the Frenchman overcame Oscar Hernandez of Spain 6-4, 5-7, 6-2.

The sixth seed will represent Agassi's toughest test to date at this event.

"He's (Grosjean) a talented player, he's effective on clay. I'll definitely have to be executing my shots and playing good," the American said.

In Friday's other quarter-finals, defending champion Tommy Haas of Germany [Images] will play Jurgen Melzer of Austria, top-seeded American Andy Roddick [Images] will meet Luis Horna of Peru, while James Blake lines up against Nicolas Lapentti of Ecuador.

On Thursday, American Blake fell behind 2-4 in the opening set and then won 10 of 11 games to brush aside Antony Dupuis of France [Images] 6-4, 6-1.

"It's amazing what happens when you stop making errors," Blake said.

"I didn't feel I was out of it at 4-2. I think he got down on himself a little at the start of the second set and that helped me." Lapentti advanced with a 7-6 ,6-2 victory over Kevin Kim of the U.S.

• Complete tennis coverage
• All you want to know about Sania Mirza