Having wallowed in a "deep, dark pit of despair" last season, Madison Keys booked her first Grand Slam semi-final spot in four years with a 6-3 6-2 upset of Barbora Krejcikova at the Australian Open.

Keys' renaissance after a dismal 2021 season has lit up Melbourne Park this year, and at a baking hot Rod Laver Arena fourth seed Krejcikova became another casualty of the American's new-found confidence and traditional firepower.

Keys told an on-court interviewer she thought she might cry.

"It means a lot," she admitted. "Last year was really hard and I did everything that I could with my team to just really reset this off-season and focus on starting fresh and new and really just starting from zero and not worrying about last year.

"Wow, that's gone well so far. I'm really proud of myself and so thankful to my team and my friends and my family for helping me through what was a really tough year last year."

Banging down back-to-back aces to claim the first set, Keys roared to a 3-0 lead in the second before a rare wobble on serve gifted French Open champion Krejcikova a break.

Unbowed, Keys unleashed a blistering forehand return winner down the line to break back and sealed the match with a thumping first serve that Krejcikova could only push long.

The 26-year-old booked last reached the Australian Open semi-finals in 2015. That year, it took Serena Williams, the eventual champion, to stop her. Williams said after the victory it was an honour to play "someone who will be number one in future".

It has not quite gone to script, with Keys now carrying a ranking of 51 and still searching for an elusive major title four years after Sloane Stephens denied her in the 2017 US Open final.

Ashleigh Barty, left, shakes hands with Jessica Pegula after her quarter-final win

Keys will face Ashleigh Barty in the semi-finals after another dominant display from the world number one.

The 25-year-old reached the last four at Melbourne Park for the second time in three years after blitzing American Jessica Pegula 6-2 6-0 in just over an hour.

Barty, who is aiming to become the first local since Chris O'Neil in 1978 to win the Australian Open, is yet to drop a set in this tournament and is unbeaten in 2022.

"That was solid tonight. "I had a lot of fun out here, I was able to serve and find a lot of forehands in the centre of the court and I was happy to take the game on."

Barty previously reached the last four two years ago, losing to eventual champion Sofia Kenin, and is hopeful of going further this time.

"I've grown as a person, I've grown as a player," said Barty. "I feel like I'm a more complete tennis player. I'm absolutely loving playing out here. It's been a lot of fun so far and hopefully there's a little bit more left."