Swiatek, Collins beat heat to reach semis, Medvedev needs cool head
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Collins swept past unseeded Alize Cornet in just 88 minutes to match her run to the last four in 2019.
But seventh seed Swiatek had to fight back from a set down and a break down against unseeded Estonian Kaia Kanepi before finally coming out on top 4-6, 7-6 (7/2), 6-3 after a marathon 3hr 1min in energy-sapping heat.
Earlier, Collins shattered French veteran Cornet's dream of making a first Grand Slam semi-final.
As temperatures nudged 35 Celsius (95 Fahrenheit), the American 27th seed came through 7-5, 6-1 to continue her resurgence after surgery 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 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 semi-final.
There she will meet the 20-year-old Swiatek, who proved too dogged as she outlasted Kanepi, who is 16 years her senior.
"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."
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.
Men's title favourite Daniil Medvedev will need to keep his cool to reach the semi-finals after a mini-meltdown in the previous round.
Should the Russian second seed get past young Canadian Felix Auger-Aliassime in the night match on Rod Laver Arena, the prize will be a last-four showdown with Stefanos Tsitsipas.
The Greek fourth seed was in rampant form as he destroyed Italian world number 10 Jannik Sinner 6-3, 6-4, 6-2.
In a two-hour afternoon romp, his progress was 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.
US Open champion Medvedev knows he will need to keep a lid on his temper, which was pushed to the limit in the last round under severe examination by serve-and-volley specialist Maxime Cressy.
"It was long and it was not easy, they were all tough sets," said Medvedev after a tetchy performance where he clashed with the umpire, complained of bad luck and screamed "It's boring!" at Cressy's relentless tactics behind his big serve.
The US Open champion admitted he was relieved to come through the most severe examination of his title credentials so far.
"If I didn't win the fourth set I would have been in a tough mental shape," said Medvedev, the de facto top seed after the deportation of defending champion Novak Djokovic on the eve of the tournament.
He faces ninth seed Auger-Aliassime who took a confidence-boosting win against the 2018 Australian Open finalist Marin Cilic in four sets in the last 16.
"It's amazing. It puts my belief even higher, I lost three times to Marin in the past, this is my first win against him," the 21-year-old said.