Sabalenka had been a hot favourite to become the first woman since Martina Hingis in 1999 to complete an Open title hat-trick at Melbourne Park.
Instead 29-year-old Keys became the oldest first-time grand slam champion in a decade with a roller-coaster 6-3 2-6 7-5 victory at Rod Laver Arena on Saturday night.
A precocious junior talent who arrived on the professional scene as a 14-year-old, Keys had been earmarked as a future superstar.
Until now, though, the American had never delivered on her immense potential.
A grand slam victory that was a long time coming, Madison Keys wasn't sure it was possible. (Joel Carrett/AAP PHOTOS)
"I have wanted this for so long, and I have been in one other grand slam final and it did not go my way and I didn't know if I was ever going to get back to this position to try to win a trophy again," Keys said.
"And my team believed in me every step of the way. They believed in me when I didn't, so thank you so much."
Set to return an equal career-high No.7 in the world following her overdue breakthrough, Keys is the oldest woman to land their maiden major since 33-year-old Flavia Pennetta famously denied Serena Williams a calendar-year grand slam at the 2015 US Open.
The emotion-charged, watershed win atoned for Keys' despairing US Open semi-final loss to Sabelenka in 2023 after she'd won the opening set 6-0 and then served for the match.
"Aryna, unbelievable playing. I'm glad I finally got you back," Keys said.
Madison Keys triumphed in an emotion-charged Australian Open final. (James Ross/AAP PHOTOS)
"But unbelievable tennis. You are always so tough to play, and we have the craziest matches. Congratulations for another amazing grand slam run."
But while Keys rejoiced, Sabalenka earned an unwanted place in tennis history as only the third women's world No.1 in the 57-year era of professional tennis to lose a grand slam final to an opponent ranked outside the top 10.
Sabalenka made an anxious start to the match, coughing up two double-faults in the opening game to gift Keys the first break.
A nervous wreck in her only previous major final, which ended in a crushing 6-3 6-0 loss to Sloane Stephens at the 2017 US Open, Keys played without fear on this occasion.
She gained a second break after another error-riddled fifth game from Sabalenka before holding serve for a 5-1 lead.
Keys, showing the strain for the first time, faltered serving for the opening set at 5-2, hitting a pair of wild forehand and backhands to offer Sabalenka an opening.
But the Belarusian couldn't take it, double-faulting to present Keys with set point before the world No.14 clinched it with a booming backhand down-the-line winner.
The Keys to victory!— #AusOpen (@AustralianOpen) @Madison_keys caps an incredible fortnight with a breakthrough Grand Slam title!She beats Collins, Rybakina, Svitolina, Swiatek and Sabalenka to claim the crown.@wwos • @espn • @eurosport • @wowowtennis • #AusOpen • #AO2025 pic.twitter.com/p2RdID6JQcJanuary 25, 2025
Both players entered the final unbeaten in 2025 after Sabalenka opened the year with victory at the season-opening Brisbane International before Keys reigned in Adelaide.
Something had to give, and it was Keys whose star shone brightest under Melbourne's bright lights.
Despite Sabalenka upping the ante to put the title match was back on level terms, Keys held her nerve.
The third set was a thriller, full of tension as neither combatant gave an inch until Keys snared a decisive break in the 12th game to seal victory after two hours and two minutes.
The world No.14 had saved match points in the semi-final against world No.2 Iga Swiatek and claimed the trophy with a run of five straight wins over seeds.
Aryna Sabalenka was dethroned as Melbourne Park queen in a thrilling three-set final. (Joel Carrett/AAP PHOTOS)
She also defeated world No.10 Danielle Collins, sixth seed Elena Rybakina and 28th seed Elina Svitolina.
"Wow, Madison, what a tournament," said a gracious Sabalenka.
"You have been fighting really hard to get this trophy, and claimed this tonight.
"Congratulations to you and your team, really well deserved and enjoy the celebration - the really fun part."
