Holder Nadal retires from U.S. Open semi-final

NEW YORK (Reuters) - U.S. Open champion Rafael Nadal retired from his semi-final against Argentine third seed Juan Martin del Potro on Friday with right knee issues.

The world number one was trailing 7-6(3) 6-2 when he decided to call it a night.

(Reporting by Frank Pingue, editing by Pritha Sarkar)