Almagro had to pull out of a match against Rafael Nadal in May after less than half an hour with a knee injury.

The Spaniard appeared to have recovered fully and made it through a four-set match in the first round, beating Cyprus' Marcos Baghdatis.

And he had battled back from a set down to Del Potro to level the match at 1-1 before he was forced out of the tournament in the third game of the third set by an apparent recurrence of his knee problem.

When the 31-year-old, who is a three-time quarterfinalist at Roland Garros, realised he would have to throw in the towel, he collapsed in tears at the back of the court.