Aug. 8, 2009 (United Press International) -- Top-seeded Andy Roddick survived two tiebreaks Friday to reach the semifinals at the Legg Mason (NYSE:LM) Tennis Classic.

Roddick, who picked up the 500th match victory of his career to get into the quarterfinals, eliminated Ivo Karlovic 7-6 (7-4), 7-6 (7-5) to set up a semifinal meeting with John Isner.

Isner advanced by defeating Tomas Berdych 6-3, 6-7 (10-12), 6-2.

The other semifinal Saturday will pit Juan Martin del Potro against Fernando Gonzalez.

Del Potro, seeded second, won without having to take the court when Robin Soderling withdrew with an elbow injury. Soderling was the runner-up at this year's French Open, losing in the title match to Roger Federer.

Gonzalez, who reached the final of this tournament six years ago, advanced with a 7-5, 6-4 victory over Tommy Haas.