Novak Djokovic felt he allowed himself to be unnecessarily distracted by the unusual tactics of Gael Monfils during their US Open semi-final.

Djokovic will face Stan Wawrinka in the men's singles final at Flushing Meadows, after coming through a somewhat bizarre four-set encounter with Monfils on Friday.

After falling 5-0 down in the opening set, Monfils gave the impression he had lost interest as he stood well inside the baseline to return serve in casual fashion.

The Frenchman's strange approach appeared to distract Djokovic, whose own form fluctuated before he completed a 6-3 6-2 3-6 6-2 victory.

Asked if Monfils got inside his head, Djokovic replied: "Well, that one or two games I allowed, yeah. It was just so uncharacteristic and very unusual to see a player in the semi-finals of a grand slam at the beginning, basically first set, just start to, you know, move around and play like that or behave like that.

"But it shouldn't be my concern. My concern is my side and what I need to do. I allowed myself to drop the concentration and to lose the momentum for a little bit, but luckily I got back."

A smiling Djokovic added: "I had different phases [in dealing with it], I must admit. I had phases when I was p***** off, phases when I was entertained by what he was doing, and phases where I was upset with myself for allowing him to - you know, whatever he does to disturb my game and my rhythm.

"So I went through it all. It was a great theatre experience."

The world number one refused to criticise Monfils, whose performance was questioned by the likes of John McEnroe.

"I was completely caught off guard when he just stood there and chipped the ball back and didn't do much," Djokovic explained. "If I would get to the net he would go for the passing shot and hit some impossible gets and balls.

"But that's Gael. That's the way he plays now. That's the way he played always. He loves to come up with a variety in his game, and the defence shots. He loves long rallies.

"It's not the first time that he comes out and just tries different shots and puts himself in a position to defend and come up with some intriguing points and entertaining points for the crowd.

"That's who he is. I think he actually played best tennis of his life on hard courts this season and the results are showing that."