Novak Djokovic has warned his rivals he is very close to peak form after reaching the US Open semifinals once more.

The world number one's unusual passage to the last four continued as he benefited from another retirement on Tuesday.

Djokovic led Jo-Wilfried Tsonga 6-3, 6-2 when the French ninth seed retired because of a left knee injury.

It marked the third time in five matches at Flushing Meadows that Djokovic has been the beneficiary of a retirement or walkover.

Despite that, and concerns over his wrist and arm, the Serbian defending champion feels like he is nearing his best.

"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," Djokovic said.

"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."

Djokovic has played just nine full sets on his way to a 10th consecutive US Open semi-final.

However, the 12-time grand-slam champion is unworried about his lack of competitive action ahead of a last-four clash against Gael Monfils.

"I have played so many times," Djokovic said.

"I was so many times in this particular situation where I had long matches on the road to the semis of a grand slam. Also had some grand slams where I didn't spend too much time on the court.

"It really just depends how you feel, how your season has been, how many matches you have played throughout the year.

"As I said, this scenario with [an] easy [run] was ideal at this stage."