Defending champion Novak Djokovic took advantage of a bizarre performance from Gael Monfils to reach another US Open final on Friday.

Monfils had not dropped a set en route to only his second major semi-final, but the flamboyant Frenchman's run came to an end on Arthur Ashe Stadium when he was soundly beaten 6-3 6-2 3-6 6-2 by the world number one.

Tenth seed Monfils resorted to gamesmanship in the first set, looking totally disinterested after losing the first five games and his antics affected a puzzled Djokovic for a short time at Flushing Meadows.

Monfils showed what he is capable of with flashes of brilliance to take the third set after the crowd voice their disapproval with his lacklustre showing, but never really looked like claiming a first win over Djokovic in 13 attempts.

The 12-time grand slam champion only had to complete two matches to reach the last four due to injuries sustained by his opponents and has yet to be given a proper test as he now prepares for a final against Stan Wawrinka or Kei Nishikori on Sunday.

Monfils looked flat from the outset in humid conditions as he racked up the unforced errors, gifting Djokovic a break when he double-faulted and found the net with another second serve to go 4-0 down in a flash.

It was all too easy for the Serb until he was clearly affected by histrionics from his struggling opponent, who stood inside the baseline on Djokovic's first serve and casually swatted the ball back as if he had thrown the towel in.

Monfils' bizarre tactics distracted the favourite, whose serve went awry as he failed to win four set points and lost three games in a row in no time at all but finally regained his composure to see out the set.

The casual Monfils had a stroke of luck to end a thrilling rally in the first game of the second set when his backhand hit the net cord and dropped over the other side to give him a 1-0 lead, but that was not a sign of things to come.

Two-time US Open champion Djokovic wasted no time in taking charge of the second set, opening up a 5-1 advantage and Monfils limped back to his chair after he went two sets down with a whimper.

Monfils suddenly came to life after he was booed by the crowd, wafting a cross-court forehand winner to break back at 2-2 in the third and Djokovic had treatment on his right shoulder before a brutal backhand from the Frenchman took the match into a fourth set.

A medical timeout for Monfils kept Djokovic waiting and the heat took its toll on both players, but there was to be no unlikely comeback as last year's champion took a 3-1 lead and responded from being broken straight back to reach a seventh final in New York.

STATISTICAL BREAKDOWN

Djokovic [1] bt Monfils [10] 6-3 6-2 3-6 6-2

WINNERS/UNFORCED ERRORS 
Djokovic – 26/27
Monfils – 36/52

ACES 
Djokovic – 1
Monfils – 11

BREAK POINTS WON  
Djokovic – 8/20
Monfils – 4/11

FIRST SERVE PERCENTAGE  
Djokovic – 64
Monfils – 54

PERCENTAGE OF POINTS WON ON FIRST/SECOND SERVE 
Djokovic – 68/53
Monfils – 62/37

TOTAL POINTS  
Djokovic – 121
Monfils – 96