Rafael Nadal admitted he felt "mentally destroyed" after the defence of his Australian Open title ended in injury and defeat by American Mackenzie McDonald in the second round.

The top seed was already in trouble at a set and a break down to unseeded McDonald when he crouched down in obvious pain clutching his left hip after chasing out wide for a forehand.

Nadal called for the trainer after limping through the rest of the game and headed off court for treatment.

A retirement seemed on the cards, with the Spaniard's wife Maria crying in the stands, but he opted to continue despite his movement clearly being impeded.

Nadal managed to hold his serve until the 11th game, roared on by fans still believing he could somehow turn things around, but it was a case of when not if McDonald would find a way through and he finished off a 6-4 6-4 7-5 victory.

Nadal revealed he had come into the match with an issue in his hip that then became significantly worse.

Mackenzie McDonald is congratulated by Rafael Nadal

He said: "It has been a couple of days like this, but nothing like today in that movement. We're going to start talking about that now, but I don’t know what’s going on, if it’s muscle, if it’s joint.

"I have history in the hip. I had to do treatments in the past, address it a little (but there) was not this amount of problem. Now I feel I cannot move.

"I tried until the end. I don’t know if in good conditions I will win the match, I will have better chances without a doubt. But at the end, that’s it. I just tried. It was not possible."

It was a hugely sad way for Nadal to exit the tournament a year after his near-miraculous run to the title when he feared his career may have been over because of a chronic foot problem.

He went on to win a 22nd grand slam title at the French Open but has found the going tough since suffering an abdominal tear and pulling out of Wimbledon ahead of the semi-finals.

The 36-year-old said: "I don’t need to talk and explain the feelings. It was not the right moment to have something like this now.

"At the end, you need to keep going. Sometimes it’s frustrating. Sometimes it’s difficult to accept. Sometimes you feel super tired about all this stuff in terms of injuries.

"In the end, I can’t complain about my life at all. Just in terms of sports and in terms of injuries and tough moments, that’s another one. I just can’t say that I am not destroyed mentally at this time, because I will be lying.

"Hopefully it’s nothing too bad. In the end it has been three positive weeks in terms of practice. So I really hope that this doesn’t put me out of the court for a long time, because then it’s tough to make all the recovery again.

"I went through this process too many times in my career and I am ready to keep doing it, I think, but that’s not easy, without a doubt."

Nadal went into a first-round clash with Britain’s Jack Draper having lost six of his previous seven matches and, although he scrapped to a four-set victory, he was some way below his best level.

The same was true in the early stages against McDonald, a 27-year-old ranked 65 and looking for his first victory over a top-five player at a grand slam.

The match was played under the roof on Rod Laver Arena on a wet day in Melbourne and the indoor conditions certainly suited the flat-hitting McDonald.

He caught Nadal cold and broke the Spaniard twice to lead 4-1 before the top seed dug in and made a fight of the opening set.

McDonald moved ahead again at the start of the second and – although Nadal responded with three games in a row – the American had just broken to lead 4-3 when the contest effectively ended.

Felix Auger-Aliassime avoided a humbling exit in a 3-6 3-6 6-3 6-2 6-2 comeback win over Slovakian Alex Molcan.

Backed as a potential champion at Melbourne Park by John McEnroe this week, the Canadian sixth seed looked like slumping out after an error-prone first two sets on Margaret Court Arena.

The 22-year-old appeared to get fired up by a complaint to the umpire about the state of the balls early in the third set, however, and suddenly was a different player, serving like a dream and imposing his game on the world number 53.

"It's quite simple, you have to put the ball in the court without missing," Auger-Aliassime laughed, explaining the transformation.

"At the end of the day, it's not rocket science. So I was just trying to put one more ball in the court and make it simple, make it work."

It was quite a contrast with the opening two sets when Auger-Aliassime overcooked his shots on both sides and sprayed 25 unforced errors, with Molcan gleefully taking advantage.

"The numbers show it but he was just way more consistent than I was in the first two sets," Auger-Aliassime conceded.

The Canadian, who lost the first set of his opening match against compatriot Vasek Pospisil on Monday, wrapped up the contest in just over three hours with a rasping forehand - the fifth time in his career he has come back to win a five-setter.

Daniil Medvedev, the man Nadal beat in the final 12 months ago, beat Australian John Millman 7-5 6-2 6-2, while third seed Stefanos Tsitsipas ruthlessly dispatched another home hope in Rinky Hijikata 6-3 6-0 6-2.

Cameron Norrie toiled away into the early hours of the morning to defeat Constant Lestienne 6-3 3-6 7-6 (7-2) 6-3.

Persistent rain in Melbourne meant the match, which was last on the schedule, did not begin until 10pm local time having been moved to an outside court.