NEW YORK (Reuters) - Rafael Nadal tried to put a brave face on it but the defending U.S. Open champion could not hide his frustration after yet another physical setback denied him the chance to add to his haul of Grand Slam titles.

It was evident early in a semi-clash with third seed Juan Martin del Potro that the world's number one player was hurting, but in true Nadal fashion he tried to persevere until retiring with knee soreness after dropping the first two sets.