Australian Open favorite Daniil Medvedev mounted a stunning comeback Wednesday to reach the Australian Open semifinal, staying on course for a second career Grand Slam title.

Medvedev, the world No. 2, looked down and out as he lost the opening two sets to Canadian 21-year-old sensation Felix Auger-Aliassime before mounting a remarkable turnaround.

The Russian saved a match point at 4-5, 30-40 in the fourth set before completing an astonishing revival to beat the ninth seed 6-7 (4/7), 3-6, 7-6 (7/2), 7-5, 6-4 in a marathon 4 hours 42 minutes quarterfinal that finished well after midnight on Rod Laver Arena.

Asked how he managed to win, the U.S. Open champion Medvedev admitted: "I have no idea.

"I just fought to the last point and manage to raise my level."

It sets up a repeat of last year's semifinal against Stefanos Tsitsipas, which Medvedev won in straight sets before going on to lose to Novak Djokovic in the championship match.

The Greek fourth seed Tsitsipas was earlier in rampant form as he destroyed Italian world No. 1 Jannik Sinner 6-3, 6-4, 6-2.

By contrast to Medvedev's struggles, Tsitsipas made serene progress in a two-hour afternoon romp halted only by rain, which caused a brief delay for the Rod Laver Arena roof to be shut and the court to be mopped dry.

The change in conditions made no difference to a dominant, flawless performance from the world number four who grasped control of the quarter-final with a break in Sinner's opening service game and did not relinquish his firm grip.

"My humility helped a lot today, I knew I was going out on the court to face a very good player," Tsitsipas said.

"I just focused on my very best shots and it paid off better than I thought.

Swiatek through

The women's semifinal lineup was completed by Danielle Collins and Iga Swiatek who enjoyed vastly contrasting wins in the last eight Wednesday.

Collins swept past unseeded Alize Cornet in just 88 minutes to match her run to the last four in 2019 and shatter the French veteran's dream of making a first Grand Slam semifinal.

But seventh seed Swiatek had to fight back from a set and a break down against unseeded Estonian Kaia Kanepi.

The 20-year-old Swiatek proved too dogged as she outlasted Kanepi, who is 16 years her senior, 4-6, 7-6 (7/2), 6-3 after a marathon 3 hour 1-minute encounter in energy-sapping heat.

"I had so many break points (nine) in the first set that I felt like I missed my chances," said Swiatek, the 2020 French Open champion.

"I was pretty annoyed and I should have been focused on the next point. And that's what I did in the second set."

As temperatures nudged 35 degrees Celsius (95 Fahrenheit), the American 27th seed Collins came through against the tenacious Cornet 7-5, 6-1 to continue her resurgence after serious medical problems last year.

"It feels incredible, especially after some of the health challenges that I have had," said Collins, 28, who had "scary" surgery for endometriosis in April last year and then tore her abdomen at the French Open.

"To be able to get back to this level and be able to compete the way I have and be as physical as I have has been so rewarding."

Collins won her maiden WTA titles at San Jose and Palermo after recuperating and has now matched her 2019 run at Melbourne Park to reach her second Slam semifinal.

Top seed Ashleigh Barty will face unseeded American Madison Keys in the other last-four clash on Thursday for a place in Saturday's final.