Three-time former champion Andy Roddick of the United States has won his 501st match to reach the semifinals of the Legg Mason Tennis Classic in Washington. Roddick beat Ivo Karlovic of Croatia to reach the final four.

In front of a sell-out crowd, Andy Roddick took advantage of Karlovic's unforced errors to win their quarterfinal in two tiebreakers, 7-6, 7-6.

Roddick was down a break in both sets to the two-meters, eight centimeter tall Karlovic. Afterwards, the American said he felt lucky to win the match.

"Getting down an early break against Karlovic is a recipe for disaster. That's absolutely not the way you want to go about managing that match. And I consider myself fortunate to be able to get out of it one time, much less two times," he said.

Roddick, a three-time champion at this event, takes on countryman John Isner, a three-sets winner over Tomas Berdych of the Czech Republic (6-3, 6-7, 6-2).

The other semifinal is an all-South American contest between defending champion Juan Martin Del Potro of Argentina and Fernando Gonzalez of Chile.

Del Potro did not play Friday when his opponent, Sweden's Robin Soderling, withdrew with an elbow injury.  Gonzalez scored a straight-sets win over German Tommy Haas (7-5, 6-4).

The 29-year-old Gonzalez owns a 3-0 record against del Potro.  But the Chilean says the 20-year-old Argentine is playing good tennis and beating him will be a challenge.

"Yeah, I haven't played for maybe two years against Del Potro," he said.  "He's a different player now. He is one of the new generation and he is having really good results in the past year. We are really good friends. I mean it is going to be an interesting match, but it is going to be fun."

This tournament is one of the key warm-ups for the last major of the year, the U.S. Open, which starts August 31 in New York.  This week's winner earns $300,000.