By Larry Fine
NEW YORK (Reuters) - Top seed Novak Djokovic advanced to the semi-finals of the U.S. Open when ninth-seeded Frenchman Jo-Wilfried Tsonga retired at the start of the third set of their quarter-final on Tuesday, trailing 6-3 6-2.
Tsonga took a medical timeout during the changeover at 5-2 of the second set and had the area below his left knee taped.
World number one Djokovic held serve at love to close out the set for a 6-3 6-2 lead but the Frenchman walked off to give the Serbian passage into the U.S. Open last four for the 10th consecutive time.
It has been a strange slam for Djokovic, who has played only two full matches in the tournament, gifted by a walkover in the second round and a retirement after six games in the third round before his abbreviated match with Tsonga.
Djokovic advanced to a semi-final against France's 10th seed Gael Monfils, who beat compatriot Lucas Pouille in straight sets. Monfils has yet to beat Djokovic in 12 career meetings.
(Editing by Steve Keating)