Federer is going into his 21st season as a professional and is targeting a 20th Grand Slam title at next week’s Australian Open.

The Swiss will start as favourite with a number of his top rivals, including Rafael Nadal and Novak Djokovic, battling injuries.

But McEnroe, a former top 50 singles player himself and younger brother of seven-time Grand Slam champion John, believes Federer’s own fitness level at the age of 36 is entirely of his own creation.

“He’s been extremely smart about taking care of himself. What he does with his training doesn’t get spoken about a lot, but he works his butt off,” McEnroe said.

“He’s changed the way he’s trained in the last five or six years to give himself more longevity. He doesn’t talk about it.

“He doesn’t want people talking about it. We all watch him and we think it’s so easy for him. That’s partly true.

“But it’s also true that he’s really mastered how to pace himself, to care of himself, train for tennis in a way that is absolutely brilliant, and also train his body and his mind.

“I was always amazed when he was in his prime, when Djokovic and Nadal started to overtake him a little bit, how he sort of brushed off losses.

“I was thinking ‘This guy is going down, never get back to No 1’.

“He basically was like ‘Yeah, it’s no big deal, I lost, I’m getting ready for the next tournament’.

“Even at the US Open this year when he lost, the first thing he said in the press conference was ‘I can’t wait to get 100% healthy so I can go to Shanghai early and be ready to win the tournament in Shanghai’.

“Lo and behold, that’s exactly what he did.

“It’s just incredible. Let’s just continue to appreciate what he does.”

Federer is already tops the all-time list of Grand Slam winners with 19 but Nadal, five years his junior, is just three behind.

Back on 14 is Pete Sampras, who dominated the sport before Federer burst onto the scene by winning Wimbledon in 2003.

But McEnroe believes Federer, who won his most recent Grand Slam title just weeks before his 36th birthday, is even more impressive than Sampras, who ended his career by winning the 2002 US Open.

“The fact that [Roger] is still able to play at this level, quite frankly, I think it’s just one of the most amazing feats I’ve ever seen in any sport,” McEnroe added.

“Sampras won the US Open and walked away, but up until that time he wasn’t even in contention at a major for a while. He got lightning in a bottle for two weeks. God bless him, he is an all-time great.

“But Federer is just a guy who is always there.

“I think he’s got a God-given gift in what he does.”