Vince Spadea advanced to the quarterfinals of the U.S. Clay Court Championships in Houston on Thursday after Australian Open finalist Marcos Baghdatis was forced to retire from their match with a sore back.

The unseeded Spadea was leading 6-2, 3-1 when the fourth seeded Cypriot wildcard walked to the umpire's chair and said he could not continue.

Baghdatis had called for the trainer several times throughout the match and received treatment on the court.

"I don't really know what it is yet," Baghdatis told the ATP website. "Maybe the nerve is squeezed.

"I just felt pain coming down through my leg. Maybe fatigue has something to do with it.

"Since Australia I've been playing all the time, working hard. I haven't wanted to stop."

American Mardy Fish [Images] also received an easy passage into the final eight, earning a walkover when Germany's [Images] Rainer Schuettler was forced to pull out of the second-round match with a stomach ailment.

Spadea will now meet Fish in the quarterfinals.

"I didn't expect to get through this way but I played a solid match," said Spadea. "I hope for Marcos' sake it's not too serious.

"Mardy has a different style of game - not a typical clay courter - so it could be a tricky match.

"I can't let him dictate too much. It will be a tough match. He's really trying to come back hard now."

Also advancing was American Paul Goldstein who disposed of compatriot Scoville Jenkins 6-4 6-1.

Goldstein will next meet Spaniard Albert Montanes, who booked his quarterfinal place with tidy a 6-2 6-3 win over Germany's Tomas Behrend.

Spain's Fernando Vicente was a straight sets 6-2, 7-5 winner over Frenchman Antony Dupuis and will take on Austrian Jurgen Melzer, who battled to a 7-6 (7-2), 6-4 decision over Brazilian Marcos Daniel.

The featured quarterfinal clash will pit top seed and defending champion Andy Roddick [Images] against another former winner and fifth seed Tommy Haas.

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