MELBOURNE, Australia -- Madison Keys said breaking through at the Australian Open for her first Grand Slam title "means the world" after she defeated world No. 1 Aryna Sabalenka in three thrilling sets Saturday.

Keys, the 19th seed, was a big underdog coming into the clash with two-time reigning champion Sabalenka, but she produced her best tennis to win in two hours and two minutes 6-3, 1-6, 7-5.

At 29, Keys becomes the second-oldest first-time women's winner of the tournament after China's Li Na broke through for her win in 2014 at 31.

The American started fast against Sabalenka, who struggled with her serve early. Two double faults helped gift Keys the opening game, and Keys held for 2-0 advantage.

While Sabalenka struggled on her serve, Keys flourished. Through two service games, she had landed all but one of her first serves to put pressure on Sabalenka, who was unable to win her way back to level and was instead broken again for a 4-1 Keys lead.

And while Keys had the chance to serve out the first set not long after, Sabalenka started to lift. She broke Keys, but the seeds of doubt didn't take root for the American, who broke back immediately. Another Sabalenka double fault -- her fourth of the set -- brought up break point and paved the way for Keys to take the first set 6-3 in just 35 minutes.

Sabalenka's four first-set double faults was her most in a single match in Melbourne, let alone in a set, while her lopsided return of just four winners and 13 unforced errors painted the picture of an uncharacteristically bad start for the Belarusian.

But Sabalenka's record coming back from a set down in Slams had been remarkable. Coming into the final, she was a staggering 10-1 in majors after losing the first set, well ahead of the next-best player in that time, Iga Swiatek, who boasts a 6-5 record.

After a quick bathroom break, three-time Slam winner Sabalenka reappeared, seemingly reenergized. Breezing through her first service game, Sabalenka converted on her third break-point chance of Keys' service game to lead 2-0.

Not long after, Keys gave up a second service game, and the reigning champion found herself up 4-1 and steaming toward taking the second set, eventually taking it 6-2 and sending it to a decider.

After exchanging 11 holds and with a tiebreaker looming large, the decisive moment of the third set -- and the match -- came with Sabalenka down 0-15 and serving to stay in the tournament at 5-6. Having already sent an off-balance forehand long, Sabalenka served wide and the American rattled off a huge backhand return winner to which Sabalenka could only grimace in frustration.

Sabalenka steadied momentarily for 15-30, but a forehand error into the net in the next point brought up two championship points for Keys. She sent one wide, but converted the second with a stunning inside-out forehand winner. Keys screamed in ecstasy as she secured her first Grand Slam title.

"I just kept telling myself, 'Be brave, go for it, just kind of lay it all out on the line.' At that point, no matter what happens, if I do that, then I can be proud of myself. It just made it a little bit easier," Keys said in her news conference.

It was her fifth three-set win at the tournament, the most in a single Australian Open since the Open era began. She knocked off four top-10 seeds (Sabalenka, Danielle Collins, Elena Rybakina and Swiatek) en route to the Daphne Akhurst Memorial Cup. It's a feat tied only by Evonne Goolagong -- who presented Keys her trophy -- who defeated four top-10 seeds at Wimbledon in 1980.

"I really felt like going into each match that if I could just try to go out, play how I wanted to play, I was really just going to give myself the opportunity to try to win the match. I felt like not stressing about things that I couldn't control, I just felt like I was able to play a little bit more free," she said.

"I think there was a confidence in maybe not playing matches amazingly from start to finish and having some dips here and there, but being able to end on a really high note each time and figure out how to get back in matches, or how to close out a match really well ... I just slowly started continuing to build the confidence.

"I think part of it was that I never really got ahead of myself in each round. I never once thought about the next round until I was actually there. So, I think, yes, I believed that I could do it. I also think I did a good job of just focusing on the task at hand."

Keys mentioned she'd been using therapy as a way to unburden herself of expectations after years on the tour, and told ESPN that "letting go" of trying to win a major is what helped her succeed in Melbourne.

"I've done a lot of work to no longer need [winning a Grand Slam]. I really wanted it, but it's no longer the thing that was going to define me and, kind of letting go of that burden, I finally gave myself the ability to play for it," she said after the win.

Meanwhile, Sabalenka lamented her poor form in the first set, saying Keys managed to push her onto the back foot with powerful groundstrokes and assertive serving.

"I think she played super aggressive. It seemed like everything was going her way. I was just trying to put the ball back. Couldn't really play my aggressive tennis and didn't feel my serve that well. The return was off. Then in the second set I kind of got my rhythm back," Sabalenka said.

"She just played incredible. It seems like she was overhitting everything. The depths of the balls were really crazy. I was trying my best. Obviously [it] didn't work well."

Sabalenka also dismissed an unusual postmatch racket smash as "frustration," saying she needed to leave the arena briefly to compose herself before the ceremony.

"I was so close to [achieving] something crazy," she said. "When you're out there, you're fighting, but it seems like everything going not the way you really want to go. I just needed to throw those negative emotions at the end just so I could give a speech, not stand there being disrespectful. I was just trying to let it go and be a good person, be respectful."

With the breakthrough major title, Keys moves up No. 7 in the world, which matches a career high she last achieved in 2016.