Andy Roddick has withdrawn from next week's Madrid Masters and is doubtful for the French Open later this month.
Tournament organisers have announced that the former world number one has a hamstring injury and has also pulled out of the Rome Masters later this month.
Given the Italian tournament starts less than a fortnight before the French Open is due to get under way, the American is facing a race against time to be fit enough to compete in Paris.
The three-time Wimbledon runner-up is likely to be more concerned about regaining full fitness in time for the grasscourt season as he has rarely impressed on the clay, his big serve being nullified by the slow surface.
Roddick's compatriot and current American number one Mardy Fish has also withdrawn from the Madrid event.
The pair will be replaced by Dutchman Robin Haase and Olivier Rochus, of Belgium, in the main draw which starts on Sunday.
Defending champion Novak Djokovic, Rafael Nadal and Roger Federer are due to compete on the new blue clay surface at the Caja Magica.
Nadal has slammed the ATP for allowing tournament organisers to change the colour of the clay, who described it as soft and slippery after practising on it earlier in the week.
The Spaniard has never been a fan of the event since it was moved from the hardcourt season and reiterated his stance on Friday.
"My thoughts haven't changed on the concept and organization of this tournament," Nadal said. "My criticism is not directed at the tournament but at the ATP, which should never have allowed such a change at a tournament of this scale.
"This is a clay-court event that is least like a clay-court event. There is always a chance of playing worse here. I only speak my mind for the good of the tour and the players, so that we have a clay tournament with few problems."