Novak Djokovic through as Mikhail Youzhny retires

World number one and reigning US Open champion Novak Djokovic has booked his spot in the fourth round of the tournament after Mikhail Youzhny was forced to retire from their third-round match in the first set.

The Serbian was handed a walkover in his second-round contest after Jiri Vesely pulled out, and now he has advanced to round four after only playing one match in this year's competition.

Djokovic, who has won the US Open on two occasions, raced into a 3-0 lead against Youzhny before the Russian held for 3-1, but signs of a thigh problem were apparent and the 34-year-old was forced to retire at 4-2 down in the opening set.

Big-serving John Isner or Great Britain's Kyle Edmund will be Djokovic's opponent in the next round.