Novak Djokovic through to US Open semi-finals as Jo-Wilfried Tsonga retires

Defending champion Novak Djokovic has made it into the semi-finals of the US Open after his opponent Jo-Wilfried Tsonga was forced to retire through injury.

The world number one was leading 6-3 6-2, requiring just one more set to make it into the final four, but it did not get that far as Tsonga was struck with a knee problem.

Djokovic, who has recently struggled with wrist and elbow injuries, dropped serve once in the clash, which ended abruptly one hour and 22 minutes in.

The 12-time Grand-Slam champion has completed just two of his five matches at Flushing Meadows following a walkover and two retirements.

The Serbian will face Gael Monfils in the semi-finals in New York.