Number one seed Andy Roddick was forced to dig deep but eventually overcame Jurgen Melzer 7-5, 7-6 to move into the semi-finals of the U.S. Men's Clay Court Championships in Houston on Friday.

The eight-seeded Austrian gave Roddick everything he could handle in the match, constantly fighting off break points to hold his serve while making several spectacular shots to keep the match going.

But the American stood firm in the wake of Melzer's inspired play to grind out the win, taking advantage of Thursday's rest day ahead of the last eight encounter.

"I definitely wanted to make him play and luckily for me, I was able to step up and hit some good shots and play some good points," Roddick said.

In other matches, seventh-seeded Luis Horna of Peru scored a 7-5, 6-3 victory over veteran American Todd Martin and will meet Roddick in Saturday's semi-finals.

Fifth-seeded Andrei Pavel upset fourth-seeded James Blake 6-4, 6-4 although the American did fight off three match points before the Romanian could finally seal his victory.

Tommy Haas of Germany defeated Dmitry Tursunov of Russia 6-2, 6-0 and will meet Pavel in the other semi-final.

"Every win is huge for me right now, giving me confidence and working the legs out there," said Haas, alluding to his return to form after a long struggle against injuries.