NEW YORK—Gael Monfils hit back against criticism of his style of play during portions of his U.S. Open semifinal against Novak Djokovic, saying he mixed up his shots in an attempt to turn the match around.

Refuting accusations that he did not give full effort, Monfils instead insisted that he showed his competitiveness by going to a style few players employ.

"I'm competing, you know," he said. "I think I'm gutsy to try that, you know, against the world No. 1."

Monfils went down 5-0 in the first set before getting back to 5-3, making heavy use of slice and unconventional off-pace shots. The Frenchman won the third set before falling in four to the No. 1-ranked Serb, occasionally hearing boos from the crowd.

"It's not only one way to play tennis, you know,” he said. “When the guy is hitting clean and you're not serving good—and actually, I wasn't returning good—yeah, you just show him. I won't win a match like that, but I can win maybe 15 minutes, maybe two more games, one more game. I can push him a little bit to defend.

Mikael Tillstrom, who coaches Monfils, said they had planned to us a variety of tactics, including slices and net play.

"We knew that we had to change something up," said Tillstrom. "And that maybe took Novak a little off guard.

He needed to stop what was happening. We had three different ones we wanted to use: slice more, bring the speed down and try to go further back.”

Tillstrom did acknowledge, however, that Monfils might have tried unusual tactics too much.

"Again, he was winning by doing that,” Tillstrom said, “so he just kept on doing it."

But the coach was pleased to see his player adjust, rather than play the same conventional game.

"[If] the momentum is for the [other] guy, you can't just keep doing it just because it looks better," he said.

Some of the plays were based on Gilles Simon's five-set loss to Djokovic at this year’s Australian Open—"We focused on that match," he said—and Roger Federer's four-set loss to him at the same tournament.

"Also, watching Roger play ... In the third set, he took a step back," he added. "Otherwise it would have been a runaway."

However, John McEnroe, commentating on ESPN, described Monfils’ untraditional play as "bordering on unprofessional."

"You know, I like John,” Monfils said. “I think he's a great person. I mean, I have nothing personal [against] him, so I'm very sad to heard that. I'm very sad to learn that such a legend [would] criticize me ... What I can say to John is, ‘You know, John, I want to be the best.’ It's tough, you know. And I try my best. I'm sorry if you think I'm unprofessional, but I guess I'm working, I'm learning, you know?

"You know, it's tough, because when he call me unprofessional, he calls my coach unprofessional, calls my physio unprofessional.”

Monfils was playing his second career Grand Slam semifinal, and first in eight years.