Spain's Rafael Nadal was given a walkover into the quarter finals of the Madrid Masters on Thursday when opponent Juan Martin Del Potro withdrew before their third-round match with a hip injury.
The unseeded Del Potro had been bothered by the problem since winning an Estoril semi final last weekend.
The 2009 US Open champion is making a recovery this season after missing more than eight months of play in 2010 after wrist surgery.
"I have experience with injuries, and I decided not to play," Del Potro said as he prepared to fly home to Buenos Aires for medical checks.
The Argentine will not play the Rome Masters starting on Sunday and could be in doubt for the French Open starting on May 22.
"The pain was very strong," said the Argentine.