MELBOURNE - Top seed Rafael Nadal's title defense came to an end after a second round defeat to Mackenzie McDonald, while women's favorite Iga Swiatek cruised through as volatile weather on Wednesday once again marred the Australian Open.

In what was a major upset on day three, Nadal was clearly hampered by a hip injury in his 6-4, 6-4, 7-5 loss at Rod Laver Arena.

It is the 36-year-old's worst result in a Grand Slam in seven years and casts further doubt on his legendary career, having struggled with a slew of injuries in recent months.

Nadal's defeat means arch-rival Novak Djokovic can draw level on his record 22 Grand Slam titles if the Serb claims a 10th Australian Open trophy.

Nadal appeared to aggravate his troublesome hip in the second set and required medical treatment. He continued playing but his movement was compromised as the 65th ranked American capitalised.

"I don't know if it is muscle or if it is joint. I have had a history with the hip," Nadal told reporters.

"I was not able to hit the backhand at all and I was not able to run for the ball."

"I did not want to retire as defending champion here."

After scorching weather then late heavy rain caused a slew of postponements on Tuesday, Melbourne's notorious temperamental weather again reared on day three with damp conditions compounding a scheduling backlog.

Men's sixth seed Felix Auger-Aliassime of Canada enjoyed a stirring comeback victory to prevail 3-6, 3-6, 6-3, 6-2, 6-2 against Slovakia's Alex Molcan.

There was no such drama for third seed Stefanos Tsitsipas of Greece and Russia's two-time finalist Daniil Medvedev, who beat Australians Rinky Hijikata and John Millman respectively to continue their ominous form.

American Frances Tiafoe ended the dreams of China's 17-year-old Shang Juncheng with a comfortable 6-4, 6-4, 6-1 victory.

Tiafoe, seeded 16th, hit 40 winners to overwhelm Shang, who was the youngest player in the men's draw.

Women's top seed Swiatek from Poland stormed past Colombia's Camila Osorio 6-2, 6-3 in a much-improved performance after her sluggish first round effort against Jule Niemeier.

"It was really intense physically and Camila didn't give me many points for free, so I needed to really work for each of them," she said. "I'm pretty happy that I won and I can play in the next round."

Third seed Jessica Pegula of the United States continued her strong form to beat Belarus' Aliaksandra Sasnovich 6-2, 7-6 (5), but sixth seed Maria Sakkari of Greece was tested against Russian qualifier Diana Shnaider before prevailing 3-6, 7-5, 6-3.

American Coco Gauff, tipped as a successor to recently retired Serena Williams, overcame 2021 US Open champion Emma Raducanu 6-3, 7-6 (4) in a clash between emerging stars.

Seventh seed Gauff, 18, held her nerve in the second set tiebreak as she made another step towards her first Grand Slam title.