Nadal had just started the fifth set decider in his quarter-final with Marin Cilic when he collapsed to the ground in agony. 

Shortly afterwards the world No 1 - favourite for the title alongside longtime rival Roger Federer - was in tears as he limped off the court. 

And a gutted Nadal appeared to be struggling to contain his emotions an hour later as he faced the world's press at a news conference. 

It is only the second time in his career Nadal has retired during a Grand Slam match.

He required a medical timeout earlier in the match for what looked like a muscle injury in his left leg. And, after being broken by Cilic for 0-2 in the final set, he decided he couldn't continue. 

Nadal said: "An opportunity is gone for me. I am a positive person and I can be positive, but today is an opportunity lost to be in the semifinals of a Grand Slam. 

“That’s already happened a couple of times in my life. It’s tough to accept.”

Speaking about his injury, he added: “It's difficult to know exactly what it is now. Difficult to know exactly the muscle. 

“I need to wait a couple of hours. Tomorrow I am going to do a test, an MRI, then we will know."

Nadal added that the hard court season, combined with five-set matches, was taking its toll on male professionals: "There are too many injuries. We need to think about the lives of players after their careers. 

"They have to think a little bit about the health of the players. There is life after tennis.

"I don't know, if we keep playing on these very, very hard surfaces, what's going to happen in the future with our lives."

The 17-time Grand Slam champion, who lost the Australian Open final to Federer last year, will likely now look towards the clay court season, with the defence of his Roland Garros crown his top priority assuming he returns to full fitness.

Cilic said after the match: “Unbelievable performance from both of us and really unfortunate for Rafa.

"He’s an unbelievable competitor and always give his best. It’s very unfortunate for him to finish this way.

"As often sometimes happens when you’re wounded sometimes the balls are going in and you are a little bit looser so I was really paying attention first couple of games just trying to keep my intensity up.

"I had several break points in this last game and it was absolutely important for me to keep that intensity and continue with my own game, not looking across the net."

Injuries have stalked Nadal throughout his career.

The 10-time French Open champion spent much of 2016 recovering from knee problems.

The sustained period off the court meant his 2017 campaign was a huge success and he claimed the French Open and US Open crowns.

Nadal finished last year as the world No 1 but crashed out of the ATP World Tour Finals due to injury.

It was hoped he would be fit to challenge for the title in Melbourne but the left-hander endured a miserable build-up to the tournament.

He came through the early rounds without dropping a set until Argentine Diego Schwartzman pushed him to four in the fourth round.

Cilic won today’s encounter 3-6 6-3 6-7(5) 6-2 2-0.

He won eight of the last 10 games as Nadal visibly weakened after suffering the injury.

And when Cilic battled to break his opponent at the start of the fifth set, Nadal knew he would never break back.