Novak Djokovic usually has all the answers on the court, but he didn't know what to say soon after his semi-final win over Gael Monfils at the US Open on Saturday morning (AEST).

In a strange match that didn't reach the heights expected of a grand slam semi, the world No. 1 eventually did enough to best the Frenchman in four sets 6-3 6-2 3-6 6-2 and book his place in the final where he'll play either Stanislas Wawrinka or Kei Nishikori.

In hot, humid conditions, both players looked to be struggling physically. Djokovic needed treatment on his shoulders and the pair often looked sluggish, not playing anywhere near their best tennis.

"Humid. That's all we were thinking about and feeling in the last couple of sets," Djokovic said afterwards.


"Conditions were difficult during most of the days during this tournament. It's the same for both of us and we expected it ... the number of you guys (the crowd) coming in made it even more electric and humid. Thankyou."

The heat was clearly having an impact on the match, and after losing the third set, a visibly frustrated Djokovic responded by ripping open his shirt.

Perhaps the heat was playing on his mind as well as his body, because a question on that very subject from ESPN's Tom Rinaldi during an on-court interview left Djokovic scrambling - unsuccessfully - to find an answer.

"There was a moment in this match where you ripped your shirt open. Why?" asked Rinaldi.

"I keep asking myself the same question," Djokovic replied to laughter from the crowd. "Sometimes you don't have an answer on this type of question, it just happens.

"We're all people and in the heat of the moment you tend to do things like that."

When asked how he bounced back from that third set to roll through the fourth and take the match, Djokovic was better prepared with a cheeky answer this time around.

"The way I bounce back from that is I take another T-shirt from my bag and I change then I feel better."

"Touche," added Rinaldi.

In a match of high drama and controversy, Monfils was jeered for an apparent lack of effort as he threatened to capitulate in straight sets. At one stage, the Frenchman even called for a can of Coke to help summon an extra level of energy.

By the end of the affair, Monfils finished with 52 unforced errors and had to save 12 of 20 break points.

In a bizarre first set, Djokovic raced to a 5-0 lead in just 15 minutes where his only challenge came from the stadium TV lights which he claimed were distracting.

However, he was even more agitated by Monfils's curious decision to stand bolt upright inside the baseline waving his racquet airily at the ball.

It looked lightweight but carried a mighty punch as the Frenchman saved four set points in the sixth and seventh games of the match.

Monfils broke in the seventh game before the Serb regained his composure to close out the set after 36 minutes.

Djokovic was quickly 4-1 to the good in the second as Monfils' interest again appeared to wane.

By the sixth game, the 30-year-old had won just nine points which he managed to nudge into double figures by the end of it.

Monfils, playing in only his second grand slam semi-final after making the last-four at the 2008 French Open, theatrically hobbled off the court and was booed.

He was roundly jeered again by sections of the 20,000-strong crowd as he dropped the first game of the third set.

But Monfils is nothing if not unpredictable and the man who squandered a two-sets-to-love lead and two match points against Roger Federer at the 2014 US Open suddenly found the on-switch.

He broke back for 2-2 and again for 4-2 as his villain's mask slipped. Djokovic provided added unexpected spice by requiring treatment on his left shoulder when he slipped down 2-5 and then saved a set point in the eighth game.

Monfils saved three break points in the ninth game before clinching the set, celebrating his lifeline with a mighty roar.

Djokovic was the more energised, breaking for 3-1 in the fourth set before double-faulting for a sixth time to hand it straight back.

The Serb then needed treatment on his troublesome right shoulder and took a medical time-out.

He broke for a 4-2 lead and this time held for 5-2 before securing the tie with his eighth break of the match.