John McEnroe has called for a major tennis rule rethink after Rafael Nadal was forced out of the tournament thanks to an injury he picked up in his match against Marin Cilic.

Nadal had been an enjoying a very strong Australian Open - seeking his first win in Melbourne since 2009 - and was enjoying a strong and competitive clash against Cilic before he started showing signs of discomfort.

Having been dragged into a fifth set, Nadal lost momentum and was clearly suffering at 0-2 down, so wisely decided to retire, allowing Cilic to progress through to the semi-final, where he will face Britain's Kyle Edmund. 

McEnroe, who was watching the events unfold, stopped short of suggesting Nadal had been using the laws of the game to his advantage, but it did prompt him to put forward a change to the current system of timeouts.

The American believes some players are guilty of exploiting loopholes within the regulations when calling for medical timeouts.

McEnroe said on commentary: "Do you have to have an injury that occurs during the match?" as per The Express.

"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 coincidental."

Coincidentally, McEnroe's comments come on the same day Micha Zverev was fined $45,000 (£32,200) - a record amount - for withdrawing from his first round match against Hyeon Chung last week.

Tennis authorities are trying to crack down on players starting matches to pick up prize money even when they are not fully fit and show little intention of completing the tie.

Regardless, the quarter-final retirement represents a big blow to Nadal, who had been considered one of the favourites to win Down Under.

Cilic meanwhile will have his sights see on adding another Grand Slam success to his 2014 United States Open victory.