Nadal, 31, led Marin Cilic in his quarter-final two sets to one before going a break down in the fourth.

As he lost momentum and Cilic hit his straps, Nadal's body seemed to be failing him and he needed extensive medical treatment.

He was limping noticeably between points and was seen wincing on court.

Nadal was clearly injured and McEnroe stopped short of saying the Spaniard was using the laws of the game to his advantage.

The world No1 has been hit by some bad luck at the Australian Open over the years and a second title in Melbourne continues to elude him.

But he did suggest a change in the way the injury timeouts work, accusing some players of exploiting loopholes.

McEnroe said on commentary: "Do you have to have an injury that occurs during the match? Are you able to say I'm a little tired can I have a rub on my legs? Is that considered a medical timeout?

"I'm sure any spot in a tennis player's body, if you press hard enough it'd hurt.

"I need to study up on this. I do feel like it's being abused - and I'm not saying it's being abused right now - but the timing of these timeouts very rarely are coincedental.

"Bathroom breaks etc. And if it's within the rules you can't blame them.

"I think they should be adjusted."

Nadal then withdrew from the match after going 2-0 down in the deciding set - handing Marin Cilic a free path into the semi-finals where he will meet Kyle Edmund on Thursday.

Greg Rusedski has now suggested a theory as to why why Nadal had to pull out the Australian Open through injury.

The Spaniard was at the top of his game last year, winning two Grand Slams and finishing as world No 1.

Before 2017 Nadal enjoyed a three-month break - enforced due to a wrist injury - while this year has been different.

He pulled out the ATP Finals with a long-standing knee problem and Rusedski feels he hasn't had time to get his body right.

The pundit told Eurosport: "Last year he had three months to prepare for the Australian Open.

"He didn't have the same preparations this year and that could be one of the reasons for the injury.

"Also, the way he moves is so physical. It's not like Roger Federer, someone who glides around the court."

What has Rafael Nadal said about his injury?

Q. How bad was the injury? How does this affect your schedule for the rest of the year?

RAFAEL NADAL: I can't say because I don't know. Just happened 10 minutes ago and is impossible to know.

Q. When did you feel it? Where exactly is it?

RAFAEL NADAL: As I said before, is difficult to know exactly what it is now. Is difficult to know exactly the muscle. Just happened minutes ago. This type of injury is difficult to know immediately, no?

We need to wait a couple of hours. Tomorrow I am going to do a test, an MRI here, then we will know. But now...

Q. Is it your thigh or your hip?

RAFAEL NADAL: I don't know. No hip. No hip.

Q. Your leg?

RAFAEL NADAL: I can't tell you exactly the muscle. It's here high.

Q. High on the leg?

RAFAEL NADAL: High on the leg. But I don't want to lie. Tomorrow we going to communicate what's going on after the MRI. You know, is not the moment to say what's going on or what not going on because we really don't know and the doctor really don't know yet. Is better to wait just a few hours. Give me that time, and tomorrow afternoon we'll let you know.