Some things in sport are just sad to see, and this was one.

An injured, ageing Rafael Nadal has been turfed in the second round of his Australian Open title defence as American Mackenzie McDonald claimed the biggest win of his career in straight sets.

McDonald won the match 6-4, 6-4, 7-5 and was leading before Nadal picked up the injury but that did not help the Spaniard.

Nadal dropped the opening set courtesy of a high level from the American and his own sloppy start, where unforced errors flowed.

Signs of life were evident in the second set, then bang.

Nadal, after hitting a backhand winner and being stretched on the forehand, felt for his side.

At 3-5 he called for the trainer and left the court with the physio for treatment.

Nadal looked inconsolable but he forced McDonald to serve out the set at 5-4 but his movement was badly hampered.

Nadal then tried valiantly to hit his way out of trouble in the third set and hit plenty of winners off the serve and the forehand, but a nervous McDonald only had to push Nadal around the court and at 5-all he broke Nadal before serving out the match to claim a massive win.

McDonald said closing out the match and the potential for ending Nadal's Australian Open career weighed on him.

"It was pretty tough to stay mentally engaged there so I am pretty happy," McDonald said.

"He is an incredible champion and is never going to give up regardless of the situation, so I am glad I was able to focus on myself and I got through."

What this means for Nadal's future is unclear. When he won the title last year he was noncommittal as to whether he would return for another Australian Open.