Gael Monfils said he was "sad" to hear former champion John McEnroe had questioned his professionalism after the Frenchman's defeat by Novak Djokovic in the US Open semi-finals.
Monfils, 30, appeared listless, at times hitting the ball at half-pace, which prompted TV commentator McEnroe to say: "You can't support that."
The Frenchman said he changed tactics to counter a fast start by Djokovic.
"I'm very sad to learn that such a legend criticised me," said Monfils.
"At the end what I can say to John is, you know John, I want to be the best. It's tough, you know. 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 a lot but I try to stand up. It's tough because when he calls me unprofessional, he calls my coach unprofessional, he calls my physio unprofessional, all my team unprofessional."
Monfils had lost all 12 of his previous matches against Djokovic but fought back from two sets and a break down to force a fourth set, eventually losing 6-3 6-2 3-6 6-2 in two hours and 32 minutes.
The 'slowball' tactics and lack of movement began after he fell 5-0 behind and by the third set drew some boos from the crowd, with four-time winner McEnroe saying: "Monfils is bordering on unprofessional.
"And the only reason I say bordering is because it's working."
Monfils said: "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, also [give] myself more confidence, and put him out of his balance.
"It was a great strategy I think."
Russell Fuller, BBC tennis correspondent: "Having lost the first five games of the match, Monfils started playing at half pace: ambling around the court and chipping back gentle returns. It was, he says, a premeditated 'Plan B', and it earned him three games in a row.
"Djokovic, however, won nine of the next 11 as Monfils' tactics endured. Whatever his intention, he gave the impression of a man not fully trying and it was an uncomfortable watch.
"Monfils won the set in which he played more conventionally, but he did not remotely look like a man who thought he had the beating of the world number one."