Washington  - Defending champion Juan Del Potro got a free ride into the semi-finals of the Legg Mason Classic Friday as Swedish opponent Robin Soderling withdrew before their match with a lingering right elbow injury.

But 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 Saturday semi-final against his fellow South American Del Potro.

The Chilean owns 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 a player who came out of the southern winter only to find unseasonably mild temperatures. "Tommy and I both played good tennis at the start, but I was able to get a break in the first set.

"In the second he gave me two break points and I took them."

But it wasn't all one-way traffic for the number 11 who stands 27- 7 on the season. Serving for victory leading a set and 5-2, Gonzalez lost serve to give Haas some breathing space.

After the 31-year-old German held, Gonzalez finally put the victory away a game later on his second match point as he smashed home the win in just under 90 minutes.

Del Potro was glad to have a free afternoon after the pullout of Soderling, elongating his practise to an hours and planning to relax for the rest of the day.

"I found out about the pullout during my warmup so I just extended the session," said the world number six South American. "I need time anyway to rest for my next match.

"The courts are playing a lot faster this year and we players can all feel the difference."

French Open finalist Soderling said he has been feeling his elbow for several weeks. "I've just over-played," said the French Open finalist who knocked Nadal out of that major on May 31 in the fourth round and lost the final to Roger Federer.

"I took a week off after Hamburg (July) but it is still bothering me, I hope to be ready for the US Open."

Haas, 31, was playing in his fourth Washington quarter-final. (dpa)