It will be a "major surprise" if Novak Djokovic does not win 25 Grand Slams, says tennis legend John McEnroe.

Only Margaret Court, Serena Williams and Steffi Graf have more, but McEnroe expects Djokovic to surpass them too.

"Djokovic is playing better than he has ever played," he told BBC Sport.

"I think he will probably win at least four or five more, obviously depending on staying healthy."

After triumphing in the Australian Open and French Open too, Serbia's Djokovic has now won all three Grand Slam titles this year.

With the Olympics and US Open coming up, he could emulate Graf's 1988 'Golden Slam', though says he is "50/50" about playing in Tokyo.

"Djokovic has put himself so far out in front of everyone in terms of his ability to embrace what he is doing - in terms of creating history - and being able to execute under a lot of stress," said McEnroe, who won seven Grand Slam singles titles.

"You're trying to break the all-time records - there is a lot of pressure. He's able to play his best tennis at this point.

"You expect that to go on for another couple of years, unless someone steps up and realises how great they are."

What defines the 'GOAT'?

For some, the 'greatest of all time' (GOAT) discussion is frivolous and does a disservice to the achievements of each player in their own right.

Looking purely at the numbers - where head-to-head records, rankings and other titles also need to be considered - does not tell the full picture.

It is not possible to quantify their different playing styles, how they have adapted their games, their physical and mental strength, nor how they have overcome the tough moments that have peppered all of their careers.

But it does provide the framework for the debate.

"It is incredible because in all other sports you have debates. Lionel Messi or Maradona? Michael Jordan or LeBron James?" said McEnroe.

"It's rare when you get three guys who are playing at the same time and it would be hard to argue they aren't the best three players ever.

"That's not just based on the number of majors, which obviously is a big thing, but their overall bodies of work. It is just amazing.

"Finishing the year as world number one is important, but people like to talk about records - in all sports, not just tennis. There is plenty to talk about with these guys.

"If someone had said to me when I was playing that there would not only be one guy to win 20 majors but there would be three, I would have said the same thing as what I said to the umpire in 1981 - you cannot be serious!"

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Djokovic was coached by Boris Becker between 2013 and 2016, with the German helping him win six of his 20 majors.

The Serb makes no secret of the fact he is motivated by the desire to break records, saying it is a "privilege" and his "primary goal".

"Novak is a great student of the history of tennis. He knows a lot about Rod Laver, Bjorn Borg and McEnroe - and a little about my history, of course," Becker told BBC Sport. "He is aware of who has done what

"That's important for him.

"Where does he get the motivation? It's only from competing against the history of the record books.

"That is similar to Roger and to Rafa. It is a sign of greatness."

Goran Ivanisevic, the 2001 Wimbledon champion, is now part of Djokovic's coaching team and thinks the debate will be over if the Serb wins the US Open in September.

"For me, Novak is the best ever. He's writing history. He's going to do it in US Open," said the Croat.

"I strongly believe he's going to do it. He's going to win all four in one year."

Winning Roland Garros & Wimbledon 'most difficult task'

Djokovic's success means he has become the first man to achieve the French Open-Wimbledon double since Nadal did so for the second time in 2010.

Since Laver achieved the feat in 1969, only three men - Borg, Nadal and Federer - had managed to replicate it in the Open era until Djokovic.

Becker, a six-time Grand Slam champion, says successfully switching from the clay courts at Roland Garros to the grass courts at Wimbledon is "the most difficult task in tennis".

"To come from a two-month clay-court season, where you have won the final in Paris and then win the Wimbledon title is incredible," said the German, who was a three-time semi-finalist at Roland Garros.

"Especially this year, where Novak had a couple of days to recover and get his senses back, and then to play on a completely different surface.

"It is almost impossible. Borg did it three times, Nadal twice, Federer once and now Djokovic - again it speaks volumes of the quality of these players."

So what makes the transition so hard?

"You always have a certain mindset when you play a tennis match - how you want to win," said Becker.

"On clay you usually win a match if you push your opponent into an unforced error - that's the mindset. Or if you outlast, or outrun your opponent.

"On grass, it is completely the opposite. You usually win a match when you hit more winners than your opponent. You go for the ace, you go for the winner, you go for the riskier shot.

"It's a completely different mindset. Added to that it is completely different movement and completely different play.

"Matches are shorter on grass, they change quicker. It is like a western shootout on grass - who shoots the quicker tends to win.

"Most find the movement very difficult on grass, we saw earlier in the week when lots of player slipped. It wasn't the grass was more slippery, it was because there is totally different movement on grass.

"Then there is court positioning - on clay you want to be four or five feet behind the baseline, on grass you are going to slip so you need to be further forward.

"It is almost a completely different game. That's what makes Novak's achievement this year so impressive."