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. Nadal is expected to undergo a scan to assess the damage.



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

GETTY Rafael Nadal's use of an injury timeout has come under the microscope

REUTERS Rafa Nadal is injured and out of the Australian Open

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.

GETTY John McEnroe has spoken out at the use of medical timeouts

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."

Rafael Nadal injury: Shocking pics as ace quits Australian Open Tue, January 23, 2018 Rafael Nadal is forced to quit the Australian Open with an apparent leg injury Play slideshow AFP/Getty Images 1 of 9 Rafael Nadal has been forced to pull out of the Australian Open due to injury

EPA Rafa Nadal injury: Out of the Australian Open