Gael Monfils admits he was left saddened after finding out tennis great John McEnroe had criticised his professionalism during his US Open semi-final loss to Novak Djokovic. 

Monfils - featuring in the semis of a major for just the second time - went down 6-3 6-2 3-6 6-2 to the world number one in New York. 

McEnroe was critical of the Frenchman's performance, with the seven-time grand slam winner saying Monfils acted "unprofessional" at times.

But Monfils fired back at McEnroe, saying he is "happy to talk" to the 57-year-old about his attitude. 

"I'm very sad to hear that," Monfils told reporters after hearing about McEnroe's on-air criticism of him. 

"I'm very sad to learn that such a legend criticises me, because at the end 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. I think I'm failing… but I try to stand up. 

"When he calls me unprofessional, he calls my coach unprofessional, calls my physio unprofessional, all my team, actually, unprofessional."

He added: "I want to [say] to John that I'm more than happy to talk with him.

"You know, [he] is someone that I always care about what he is saying, but I'm just sad that he criticises me that easily."