Defending champion Novak Djokovic is through to the US Open semi-finals after a knee injury forced Jo-Wilfried Tsonga to quit when trailing 6-3 6-2. The frenchman was down just when he was making a comeback into the match. Djokovic, 29, has completed just two of five matches in New York, benefiting from a walkover and two retirements.

DjokovicHe will face another Frenchman in the last four after Gael Monfils beat compatriot Lucas Pouille 6-4 6-3 6-3. “This was the scenario that I needed and I wished for,” said Djokovic, who has faced his own health issues.The Serbian world number one arrived at Flushing Meadows nursing injuries to both his left wrist and right elbow. It was a strong start by Djokovic, as he rattled off three quick points before missing on an inside-out forehand wide. A double-fault made it 40-30, but Tsonga missed on a backhand and the world No 1 was on the board first. Tsonga cameback with  two quick points, then crushed a 134mph serve down the middle that put Djokovic on the back foot and forced him into a backhand error.

At 15-all, Djokovic ripped a cross-court forehand winner from the middle of the court and moved within two points of the first set. Tsonga pushed a backhand into the net and Djokovic took the opener in just 25 minutes. During break time Tsonga was off the field and the medical team were attending him for a possible ligament tear. The break gave the French player to regroup and he came back strongly in the second set. Tsonga forced Djokovic into an error and then closed out the hold with an ace. Both players were a bit off their best.Tsonga made yet another unforced error his 24th of the match and 14th from the backhand side. Djokovic made it look easy as he took the second set when Tsonga made 35th unforced error of the match. Tsonga retired with a knee injury which troubled him right from the beginning.

Djokovic made it to the semifinals after playing 84 games.It took him nine complete sets to make it to the last four.