NEW YORK — For quite a while, Novak Djokovic’s opponent in the U.S. Open semifinals, Gael Monfils, looked as if he didn’t want to win – or even be there at all Friday.

That premeditated “great strategy” of hoping to lull the No. 1 seed and defending champion into complacency and mistakes, as Monfils described it later, worked briefly, yet didn’t prevent a two-set deficit. So he transformed back into his entertaining, athletic self. A sweat-soaked Djokovic sought help from a trainer for aches in both shoulders and what was no contest suddenly was one.

Monfils forced a fourth set and Djokovic ripped off his white shirt angrily a la “The Incredible Hulk.”

The ultimate outcome was only briefly in the balance, though. Djokovic regained the upper hand, as he so often does, reaching his 21st Grand Slam final and seventh at the U.S. Open with an eventful and at times bizarre 6-3, 6-2, 3-6, 6-2 victory.

“Well, it was a strange match,” said Djokovic, who will face No. 3 Stan Wawrinka in Sunday’s final, “as it always is, I guess, when you play Gael, who is a very unpredictable player.”

Never more so than on this muggy afternoon, with the temperature at 90 degrees and the humidity above 50 percent. Monfils, now 0-13 against Djokovic, spent most of his news conference defending his unusual approach and said he knew beforehand he might try it.

On ESPN’s telecast, John McEnroe blasted the 10th-seeded Frenchman for lack of effort. The Arthur Ashe Stadium crowd jeered him.

“First question is, like, ‘You’re not competing?’ … Yes, I’m competing,” Monfils said, cursing for emphasis. “I made a sign to my coach (to) say, ‘OK, I’m going to Plan B.'”

Djokovic had three set points while serving at 5-1, 40-love, and Monfils transitioned into something that at first blush appeared to be tanking – losing on purpose, for who knows what reason – but which he explained afterward was the tennis equivalent of Muhammad Ali’s boxing “rope-a-dope,” absorbing someone else’s best shots and pretending to not be interested in attacking.

Instead of his usual crouch, preparing to return serves, Monfils casually stood upright at the baseline, looking like someone waiting to place his takeout espresso order. During points Monfils would hit slices or make halfhearted, half-swinging strokes, then occasionally wallop a 100 mph passing shot.

“For sure, people are not ready to see that,” Monfils said. “Definitely I try to get in his head, try to create something new for him to see.”

Somehow the tactic was effective for a short while.

“I was completely caught off-guard,” Djokovic said.

Miscue after miscue arrived from Djokovic, and Monfils won three straight games in a row, before eventually dropping a set for the first time all tournament.

“I thought, at times that he was maybe behaving a little bit unacceptable,” Djokovic said. “But I guess that was part of his tactics. If he said that you have to believe him, I guess.”

Djokovic will try for his third U.S. Open championship and 13th major trophy overall against Wawrinka, whose first final at Flushing Meadows came via a 4-6, 7-5, 6-4, 6-2 victory over No. 6 Kei Nishikori that lasted more than three hours. Djokovic has won 19 of 23 previous meetings, but Wawrinka did win their 2015 French Open final.