Washington, Aug 8 (DPA) Top seed Andy Roddick will face off with compatriot John Isner for a place in the final of the Legg Mason Classic tennis tournament here after defeating Ivo Karlovic 7-6 (7-4), 7-6 (7-5).

Isner beat Czech eighth seed Tomas Berdych 6-3, 6-7 (10-12), 6-2 here Friday night.

The semi-finals in the American capital will feature a bill which also includes an all-South American clash between fourth seed Fernando Gonzalez and defending champion Juan Del Potro of Argentina.

Del Potro got a free ride as Swedish opponent Robin Soderling withdrew before their match with a lingering right elbow injury.

Roddick is seeking redemption as he returns to tennis for the first time in five weeks after losing a third Wimbledon final to Roger Federer.

Fourth seed Fernando Gonzalez had to work for his hard-hitting win, firing 18 forehand winners to overwhelm German tenth seed Tommy Haas 7-5, 6-4.

The veteran Chilean, who took four weeks off after a poor Wimbledon to rest a knee plagued with the same patella tendon problem that afflicts Rafael Nadal, showed no signs of rust as he moved into a semi-final against his fellow South American Del Potro.

The Chilean leads the Argentine, winning all three of their previous matches, the last at the Australian Open in 2007.

'I barely hit a ball when I was at home,' said Gonzalez, who said that the change of surface from clay to grass two months ago gave him more pain in his knee.

'I was also scared of the hot weather here,' added the player who came out of the southern winter only to find unseasonably mild temperatures.