Roddick is trying to win the Washington title for the fourth time

Top seed Andy Roddick continued his fine form with a straight-sets win over fellow big server Ivo Karlovic to reach the semi-finals in Washington.

The American came through 7-6 (7-4) 7-6(7-5) to set up a clash with compatriot John Isner at the Legg Mason Classic.

Isner, another huge server, defeated eighth seed Tomas Berdych 6-3 6-7 (10-12) 6-2.

Second seed and defending champion Juan Martin del Potro will face fourth seed Fernando Gonzalez in the second semi.

Argentine Del Potro progressed without hitting a ball after fifth seed Robin Soderling was forced to withdraw because of a right elbow problem.

Being down an early break against Karlovic is a recipe for disaster Andy Roddick

Gonzalez enjoyed a fine 7-5 6-4 win over Wimbledon semi-finalist Tommy Haas, and the Chilean said: "Both of us played really good tennis in the beginning.

"I only had one break point and I did it. That gave me a lot of confidence for the rest of the match."

It was no great surprise that Roddick needed two tie-breaks to see off Karlovic, but there were breaks of serve in the match.

The American netted a volley to go down 1-3 in the first set but broke back in game seven and took control of the ensuing tie-break from the outset.

Karlovic then broke again in the opening game of the second set, with Roddick hitting a ball out of the stadium in frustration before regaining his composure to quickly get back on level terms.

Roddick got the crucial mini-break at 4-3 in the second tie-break and saw it out, saying afterwards: "Being down an early break against Karlovic is a recipe for disaster.

"I consider myself fortunate to get out it of it one time, much less two times.

"If you would have told me I would have broken twice I would have thought I would have been a lot more comfortable than that."

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