World number one Novak Djokovic scrapped his way into the U.S. Open final for a seventh time on Friday with a confounding 6-3 6-2 3-6 6-2 win over enigmatic Frenchman Gael Monfils.

Often viewed as more showman than competitor, the 10th-seeded Monfils tested the patience of both Djokovic and the fans with a display of tennis that swung wildly from dynamic to indifferent.

In what was one of the more bizarre encounters to unfold on a grand slam court, Monfils was showered with boos one set and cheers the next before exiting Arthur Stadium to a mixture of both.

"He's very entertaining to watch," said Djokovic, who will play either Swiss third seed Stan Wawrinka or sixth-seeded Japanese Kei Nishikori in Sunday's final. "He's a very charismatic guy and he had a great tournament.

"It was a strange match, as it always is when you play Gael, who is a very unpredictable player. He loves to come up with a variety in his game."

Djokovic has described Monfils as the only player he would buy a ticket to watch and the Frenchman delivered an entertaining if unconventional display that drew both anger and applause.

What some viewed as 'tanking', Monfils described as tactics, a well-thought-out 'Plan B' employed when it became clear a more traditional 'Plan A' approach to the match was not going to get him past the Serb.

Djokovic had come out on top in all 12 of their previous meetings.

"Definitely I try to get in his head, try to create something new for him," explained Monfils. "And that was it. When the guy is too good, playing clean and you're playing not that good, you need to change.

"You just don't want to see it. We can change a little bit. It's not only one way to play tennis. I know it is not natural because first question is you're not competing. I'm competing.

"The change takes guts. It was a great strategy, I think."

Having advanced to the semi-final without dropping a set, Monfils did not wait long to surrender his first as he went 5-0 down in just 16 minutes.

INCREASING IRRITATION

Looking ready to throw in the towel, Monfils flapped at a few shots then sprinkled in a few brilliant returns to leave an increasingly irritated Djokovic off-balance and the crowd scratching their heads.

The Serb, however, would quickly get back into rhythm, clinching the first set and then going on to demolish Mofils in second.

But when trailing 2-0 and a break down in the third set, the impressive Monfils suddenly reappeared and he went on to take the set as Djokovic ripped his shirt and stalked the court.

Monfils' tactics, while bold, ultimately proved unsuccessful as Djokovic's skill and familiarity with the Frenchman's playing style won out.

"I had phases when I was entertained by what he's doing and phases where I was upset with myself for allowing him to do whatever he does to disturb my game and my rhythm," said Djokovic. "He's one of the most charismatic guys on the Tour.

"You know, sometimes, as his opponent it's not easy to handle his up and downs but he's a very important asset to our sport. He brings that joy."