World number one Novak Djokovic has warned his rivals that he is close to reaching his peak after he breezed into the semi-finals of the US Open.

Djokovic, 1.85 to win the tournament, was able to walk into the final four after Frenchman Jo-Wilfried Tsonga pulled out of their match due to a left knee injury.

At the time of his withdrawal, Djokovic was leading 6-3 6-2 at the Arthur Ashe Stadium and he will now take on Gael Monfils for a place in the final.

Monfils had beaten Lucas Pouille 6-4 6-3 6-3 earlier in the day but Djokovic insists he is feeling good again after a shaky start to the tournament.

At the start of last week he admitted he was still struggling with injuries but he has now seen Tsonga, Jiri Vesely and Mikhail Youzhny withdraw while playing.

With rival Andy Murray playing over three hours more tennis than him, Djokovic admits the retirements have helped him reach his peak at the right time.

He said: “I am not really concerned. “Actually, in this stage of the season, considering some physical issues I have had in the last month, month and a half, this was the scenario that I needed and I wished for.

“I got a lot of days off and recovered my body. Right now I’m feeling very close to the peak. That’s the position where I want to be. This scenario was ideal at this stage. I’m reaching my peak in terms of my form.”