Iga Swiatek is ready to go the distance in her quest for a second grand slam title, saying she has physically evolved since her shock Roland Garros crown two years ago.

The Polish seventh seed booked an Australian Open semi-final showdown with American Danielle Collins courtesy of an epic 4-6 7-6 (7-2) 6-3 victory over Estonian veteran Kaia Kanepi that lasted more than three hours.

Swiatek said she was looking to extend the rallies in the third set, confident she had the physicality to outlast her 36-year-old rival.

She also needed three sets to overcome Sorana Cirstea in the fourth round.

"I know that I'm physically really prepared, and I hoped that she's going be more tired at the end," Swiatek said.

"I actually wanted to prolong some rallies to make her more tired, because I have big trust in myself in terms of my physical shape.

"That match showed that actually it's smart to trust myself in that matter."

Swiatek saved eight set points before losing the first set and then refused to surrender on match point, scrambling to return multiple would-be winners.

"I wasn't even thinking a lot, I was just running," the 20-year-old said.

American Collins had a far easier time over French veteran Alize Cornet, winning 7-5 6-1 in less than half the time on the court.

Hard-hitting Collins, the 30th seed, made the final four at Melbourne Park in 2019 which is her best grand slam result.

She is the second American into this year's last four, joining Madison Keys.

Collins revealed this week her career had been on the up since undergoing surgery in April for endometriosis, with the condition leaving her in harrowing pain during tournaments last year.

She was delighted to be back on the main stage in Melbourne at the pointy end of the tournament.

"It feels incredible, especially after some of the health challenges I've had," Collins said.

"To be able to get back to this level and be able to compete like the way I have been and being able to be as physical as I haven't been so rewarding."

Swiatek felt she had the weapons to topple hard-hitting Collins, whose serve proved a handful for Cornet.

The French 32-year-old may have made her last singles appearance at Melbourne Park after saying she's considering retirement.

Swiatek meanwhile is ready for whatever Collins throws at her.

"I will approach it the same as any other match - I have played with some heavy hitters on this tournament already, so I feel like I'm feeling their game on my racquet pretty well," Swiatek said.

"The two matches showed me that even in tough moments I can come back, and I have the skills to win matches even when they are really hard.

"It's going to be hard and she's in great shape and really confident but I also feel that way.

"I just hope it's going to be a good match."