US top seed Andy Roddick surrendered early service breaks but beat Ivo Karlovic 7-6 (7/4), 7-6 (7/5) in a showdown of the ATP's top ace smashers on Friday to reach the Washington Classic semi-finals. Fifth-ranked Roddick, now 5-1 against Karlovic, allowed his first two career service breaks to the lanky Croatian, but broke back each time and completed his third consecutive two-tie-breaker triumph over the ATP ace leader. 'Getting down an early break against Karlovic is a recipe for disaster. I consider myself fortunate to get out of it one time much less two,' Roddick said. '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.' Roddick, playing his first US Open hardcourt tuneup after losing to Roger Federer in the Wimbledon final, fired only seven aces to 13 for Karlovic but connected on 74 percent of first serves to 59 percent for Karlovic. 'I was playing aggressive and returned well,' Karlovic said. 'I was close to a win even though my serve percentage was really low. Tactically I was doing well reading his serve but my percentage was too low. That's why I lost.' Three-time Washington champion Roddick blamed the impact of cool night temperatures on the ball for the pair's atypical serving struggles. 'I figured just put it in play and take my chances from the back of the court,' Roddick said. Roddick next faces another tall power server in countryman John Isner, who defeated Czech eighth seed Tomas Berdych 6-3, 6-7 (10/12), 6-2. Roddick downed Isner in the 2007 Washington final in their only prior meeting. Defending champion Juan Martin Del Potro of Argentina meets Chilean fourth seed Fernando Gonzalez in the other semi-final. Gonzalez ousted Wimbledon semi-finalist Tommy Haas 7-5, 6-4 while Del Potro, the second seed, advanced by walkover when Swedish fifth seed Robin Soderling withdrew from the 1.4 million-dollar event with a right elbow injury. 'It's frustrating to leave like this,' Soderling said. 'My elbow started to bother me two weeks ago and this morning it was really sore.' Roddick, the 2009 ATP leader in tie-break and hardcourt wins, extended his tie- break mastery over Karlovic after being broken in the fourth game of the first set and first game of the second set, answering each three games later. 'Every shot is magnified in a tie-breaker,' Roddick said. 'The more chances you give someone to let the moment get a hold on them, the better it is. It's not like I'm unfamiliar with the situation.' Karlovic, who also lost to Roddick in the 2007 Washington semi-finals and 2005 Washington quarter-finals, had a simpler explanation. 'Luck. Simple as that,' Karlovic said. 'I hit two nets. Each was his advantage. It was luck.' 'Luck definitely plays a part,' Roddick said. 'I certainly don't think if we played eight more in a row I would win all eight. I don't know. I wish I had a smarter answer. That's just how it rolls.' Gonzalez, off the past month with right knee tendinitis, is 3-0 against Del Potro but South America's top players have not faced each other since the second round of the 2007 Australian Open. 'There's always a rivalry between Chile and Argentina but we're good friends so it's going to be interesting,' Gonzalez said. 'He's a different player now.' 