MELBOURNE, Australia - Four months after the rout, Coco Gauff got her revenge - and the biggest win of her career to date, stunning No.3 seed Naomi Osaka 6-3, 6-4 to reach the fourth round of the Australian Open.

Read more: See how the match unfolded with the WTA Insider Live Blog

Their meeting at the same stage of the US Open in September had been an abrupt dousing of Gauffmania, with Osaka needing only 65 minutes to run out a 6-3, 6-0 winner. But today, the defending champion was undone by 30 unforced errors, while the American maintained a first serve percentage of 75% as she turned the tables in 67 minutes. Gauff, who also made the fourth round at Wimbledon last year, becomes the first player to reach two Grand Slam second weeks as a 15-year-old since Martina Hingis, who reached a US Open fourth round and semifinal, an Australian Open quarterfinal and a Wimbledon fourth round in 1995-96 before her 16th birthday. In addition, the teenager notches up her second career Top 10 win following her defeat of Kiki Bertens in the Linz quarterfinals in October.

"I learned a lot from that US Open," revealed Gauff afterwards. "Not even just playing against Naomi, but the whole tournament. I think I was on edge, and I had the three-setters in the first two rounds, and I... was just dealing with all of the media. I feel like now I'm more playing, just having fun. And, I mean, winning is a cherry on top, but I'm honestly having a lot of fun on the court, even in those tight situations."

There were also technical lessons the youngster had taken from the New York loss. Admitting that she "wasn't really prepared" for Osaka's pace in Flushing Meadows, Gauff said proudly: "Today I definitely showed that I worked on that in the off-season."

Despite the high standards Gauff sets for herself, she also revealed why she found it easier to play for fun rather than necessarily the win today. "Playing Naomi, going in today, I just knew it would be a good match," Gauff said. "She's a player and a good person, and win or lose, I think I was going to be satisfied with today's match - just because when you play someone like that and play a person like that, you can't be too disappointed after a loss, because she's been so nice to me."

For Osaka, though, the warm mutual feelings were little comfort. "I love her, but I don't like this feeling of losing to her," said the two-time major winner. "You don't want to lose to a 15-year-old, you know. But I guess that's, for me, a reality check. It doesn't really matter the age of the opponent. Of course she deserves to be here. She played her matches. I just have to work harder."

Initially, it was the Japanese No.1 placing her opponent under pressure, unleashing her forehand in Gauff's opening service game to carve out a half-chance at deuce. But Osaka was unable to reach break point - and thereafter would only win four more points on the 15-year-old's serve for the rest of the set.

Osaka would hold her own through the seven consecutive holds that kicked off the contest, but her tally of frequently careless unforced errors was rising - and in the latter stages of the set her backhand in particular deserted her, with that stroke finding the net on three straight points to concede the break for 3-5, and another two times as Gauff closed the opening act out to love.

Afterwards, Osaka lamented both her technical execution and her mentality. "It was one of those days where I couldn't do anything right," she said. "My backhand, which is my more consistent side, every ball was flying. Even though I was telling myself, I know what to do in order for it to go in. And it just wasn't going in."

The Osaka and Beijing champion identified her perfectionism as a key weakness. "I don't really have the champion mentality yet," she admitted. "Someone that can deal with not playing 100%, you know. And I have always wanted to be like that, but I guess I still have a long way to go."

The former World No.1 continued to be at sea as she dropped serve again to open the second set, glancing despairingly to her box after the netted volley and long forehand that sealed the break. Though Osaka gathered herself for a mini-revival, breaking back immediately thanks to judicious net approaches (and a pair of Gauff double faults), the respite would only be temporary.

Indeed, that would be the only sign of vulnerability that Gauff evinced on serve today. The Linz champion would otherwise keep Osaka at bay with brutal efficiency when she stepped up to the line - as well as displaying the razor-sharp competitive instincts and stellar defensive work that have become her trademarks.

As in the first set, the Osaka backhand collapsed in the key moments of the second. Four more errors from that wing conceded her serve to fall behind 3-4 - and this time, Gauff wouldn't blink. The World No.67 did not drop another point on serve as she closed out the upset with two consecutive love holds.

"Even before the tournament I think almost every player has a belief that they can win it," she mused about her uncommon levels of self-confidence. "Some stronger than others. I just always have the belief I can win regardless of my opponents."

2020 Australian Open highlights: Gauff ends Osaka title defense