A resurgent Madison Keys said Tuesday she was a different person to the one that lost an Australian Open semi-final in 2015 and was now more prepared to go all the way.

Unseeded American Keys, ranked 51, pulled off another big upset to beat French Open champion and fourth seed Barbora Krejcikova 6-3, 6-2 on Rod Laver Arena and make the last four.

Her reward is a clash with either world number one Ashleigh Barty or fellow American and 21st seed Jessica Pegula for a place in the final.

The last time Keys got this far at Melbourne Park she was just 19 and came face-to-face with a dominant Serena Williams, the eventual champion.

"I would say it mostly feels different because I'm seven years older and it's not my first semi-final of a Slam," she said.

"I think I'm a little bit more prepared this time around than I was all those years ago.

"It's also a completely different situation, time and person, all of that. I think you take the experience that you have from those moments and you try to apply it, but you also know that it's going to be a completely different challenge."

That 2015 result helped propel her into the top 10. She went on to be a 2017 finalist at the US Open and made the last four at Roland Garros a year later.

But she endured a frustrating 2021, winning just 11 matches in the entire season, a mark she has now matched less than a month into the new year after winning a lead-up event in Adelaide.

The big-serving American came into the tournament as the WTA aces leader and signalled her return to form by beating 2020 champion Sofia Kenin and eighth seed Paula Badosa en route to the quarter-finals.

- 'Proud of myself' -

"It means a lot," said Keys. "Last year was really hard.

"I did everything that I could with my team to just reset in the off season and focus on starting fresh for the new season, really starting from zero and not worrying about last year.

"Wow, that's gone well so far. I am really proud of myself."

She slammed down her first service game to love to take charge, but Krejcikova worked two break points on the next.

The American saved them both and served out with an ace but the Czech star earned another four on the following service game, only to be denied again by the gutsy 26-year-old.

Keys attacked Krejcikova back, working five break points and converting when her opponent slapped a forehand wide to give her a 4-2 lead.

The fourth seed called the trainer at 2-5, red-faced and struggling with the heat. She appeared to have her temperature and blood pressure taken.

But Krejcikova soldiered on and, though she held serve, she lost the set in 50 minutes.

With her confidence down, Krejcikova was broken twice early in the second set as Keys looked the fresher.

The fourth seed refused to give up and broke back for 1-3, but it was a short-lived fightback as Keys stormed to the finish line.

"I train in Orlando in the summer which I think is the hottest place on earth in the summer," Keys said of handling the heat better than Krejcikova.

"I am pretty used to it, just because I train in that all the time which I think is definitely a bonus for me."