MELBOURNE: Rafa Nadal's Australian Open title defence lay in ruins on Wednesday after an injury-plagued defeat that shook up the men's draw, even as rain wreaked further scheduling havoc at Melbourne Park on day three.

A wincing Nadal suffered a left hip problem midway through his 6-4 6-4 7-5 loss to American Mackenzie McDonald at Rod Laver Arena and gloomily played out the match in pain.

The result not only opens up the top half of the draw, it also hands Novak Djokovic a boost as the Serb hunts a 10th title at Melbourne Park to draw level with Nadal's 22 major triumphs in the Grand Slam race.

"It has been like this a couple of days," said Nadal of his injury.

"I have a history of hip issues.

"I've had to do treatments in the past but it was not this amount of a problem."

For all Nadal's struggles, hard-hitting McDonald was hugely impressive and had the better of the match before his veteran opponent broke down.

Humiliated by Nadal at the same stage of the 2020 French Open, their only previous meeting, McDonald was glad to square the ledger.

"Last time I played him was at Chatrier, he kicked my butt," said world number 65 McDonald.

"He's an incredible champion, he's never going to give up regardless of the situation so even closing it out against a top guy like that is always tough."

Another shock appeared set to rock Melbourne Park when Felix Auger-Aliassime went two sets down against Alex Molcan at Margaret Court Arena but the Canadian sixth seed rallied to close out the match 3-6 3-6 6-3 6-2 6-2.

UNDER THE ROOF

There was less tension in the women's side, as Iga Swiatek, hot favourite for the women's title, overcame Camila Osorio 6-2 6-3 under the roof on Rod Laver Arena.

Men's dark horse Jannik Sinner also swept into the third round, waltzing past Tomas Etcheverry 6-3 6-2 6-2 on the similarly protected John Cain Arena.

Rain kept the players off the outer courts throughout the day session, however, adding to fixture congestion triggered by extreme heat and storms on Tuesday when nine matches did not get started and two could not be completed.

Greek sixth seed Maria Sakkari had a scare on Margaret Court Arena against 18-year-old Diana Shnaider and was forced to come from a set down to beat the Russian teenager 3-6 7-5 6-3 over more than two 1/2 hours.

"It was a very high level from both of us, she played an amazing match, she's very talented, very promising," said Sakkari, before joking that Shnaider should consider giving up her college eligibility in the United States and turn professional.

Sinner, who has reached the quarter-finals of all four Grand Slams but never gone any further, could hardly have shown better form as he briskly dismissed Argentine Etcheverry.

Strong and aggressive, the 21-year-old fired 32 winners and converted all five of his break points to set up a third-round meeting with Lloyd Harris or Marton Fucsovics in an hour and 44 minutes.

"For sure, the level today was good, I served well, I returned good as well as I think he is a very good server so I'm very happy to be in the next round," Sinner said.

"I'm very happy to play on this court with the roof, hopefully it won't rain in the next days."