A little known American has knocked defending champion Rafael Nadal out of the Australian Open in one of the biggest ever upsets in the grand slam's history, sparking emotional scenes in the world number two's player box.

Californian Mackenzie McDonald, ranked 65th in the world outpowered the 22-time grand slam winner to clinch a shock straight-set victory, 6-4, 6-4, 7-5 to progress to the third round at Melbourne Park. 

A visibly distressed Nadal appeared to injure his left hip area late in the second set and left the court for treatment before returning to lose in straight sets.

The shock loss sparked emotional scenes in his player's box with Nadal's wife Xisca, breaking down in tears as he struggled to move around the court.

Footage from a spectator in the crowd behind the player's box showed a member of Team Nadal consoling Xisca by placing an arm around her.

The loss has sparked speculation it may have been the last time we'll see Nadal in action at Melbourne Park.

Now 36 and a new father, Nadal hasn't been the same since his 11th hour shock withdrawal Wimbledon semi-final last year against Australian Nick Kyrgios.

The shock loss has stunned the tennis world and sent fans into meltdown.

 'Wow Nadal gone early showing his age,' one fan tweeted.

One woman added: 'This is pretty unusual. Team Nadal have had their fair share of injuries but his wife is crying. Hopefully he has another comeback in him.'

Fans were divided over whether Nadal's time at the top is over.

'Abdominal, knee, foot etc. what else. he should just retire if everything hurts. it's a huge disrespect to the opponents if every time you complain of injury in every lost,' one fan wrote.

Others leapt to his defence.

'Rafael Nadal is the epitome of grace and humility. Even in the face of adversity, he never dies. He carries on and fights!! A true role model,' one fan tweeted

Even before second set, injury, Nadal was struggling against the contrastingly mobile McDonald, but thereafter his movement was clearly restricted and it seemed only a collapse from his opponent would save him.

Nadal was struggling to stay in the second at 3-5 when a relatively innocuous movement on the baseline saw him suddenly pull up and clutch his lower left side. There was no immediate public diagnosis but the problem area looked to be around his lower left ab and hip.

Having decided to carry on there were discussions between the player and his box, with anxious looks all round. He managed to stay competitive with limited mobility and carefree swipes at the ball, but McDonald broke him at 5-5 in the third and there was little resistance in the last game.

Nadal has now come off worst from seven out of his last nine matches going back to the US Open, a ratio of defeats not equalled since right at the start of his career.

Nadal grabbed at his left hip during the eighth game of the second set of his clash with McDonald after he appeared to suffer the injury as he planted his foot when sprinting to make a shot, immediately going off court for medical attention.

The 36-year-old resumed playing on Rod Laver Arena but was clearly in pain and struggling to run as McDonald closed out the biggest win of his career 6-4 6-4 7-5.

Nadal's box, including wife Maria who wiped away tears, was left shattered by the injury as the 22-time major winner seemingly approaches the end of his glittering career.

Even before the injury, Nadal was rattled by McDonald, who broke serve in the opening game of the match.

Nadal became frustrated, arguing multiple times with the chair umpire.

The apparent hip problem marks the sixth time in his grand career that either injury or illness has cruelled his Australian Open campaign.

He retired in the 2010 quarter-finals against Andy Murray with a knee injury and was badly hindered by a hamstring strain the following year in a last-eighth loss to countryman David Ferrer.

Nadal withdrew from the Open in 2013 with a stomach virus and suffered a back injury in the warm-up before losing the 2014 final to Stan Wawrinka.

In 2018, the great Spaniard quit his quarter-final against Marin Cilic with a hip issue.

Nadal last year finally won a second Australian Open title, 13 years after his first, by coming from two sets down to defeat Russian Daniil Medvedev in a classic decider.

The world No.2's early exit is another blow for Australian Open organisers, with the draw already ravaged by major withdrawals and retirements.

Women's titleholder Ash Barty, seven-times champion Serena Williams, her sister Venus, dual winner Naomi Osaka, world No.1 Carlos Alcaraz and the legendary Roger Federer are all missing.

Tennis Australia boss Craig Tiley was captured with a grim look on his face during Nadal's medical timeout.

Following his huge upset, McDonald was hugely respectful of Nadal and hailed his willingness to fight on despite his injury.

He said: 'He's an incredible champion. He's never going to give up so closing it out against a guy like that is tough.

'I'm happy I kept focusing on myself and got through.'