Defending champion Roger Federer loses in four sets to Stefanos Tsitsipas in the fourth round of the Australian Open. (1:21)

MELBOURNE, Australia -- As Stefanos Tsitsipas stood in the middle of the court, hands on hips in disbelief, the sellout crowd inside Rod Laver Arena wasn't sure how to react. They cheered for the victor but couldn't contain their disappointment as Roger Federer waved goodbye to this year's Australian Open.

In front of the big screen at Garden Square -- the hot spot for those who aren't lucky enough to have center court tickets -- the fans had equally mixed emotions, though it was impossible not to notice the Greek contingent that chanted wildly throughout the match. Was this upset really happening?

It was.

On Sunday night, Federer's 6-7 (11), 7-6 (3), 7-5, 7-6 (5) loss to Tsitsipas capped off a day already shaken by a series of surprising results. But this loss was different. This was Roger Federer.

"I'm the happiest man on the Earth right now," Tsitsipas said on the court, unable to fully collect his thoughts amid all the emotions. How could he not be?

"You're watching the changing of the guard," said John McEnroe, who was conducting the interview.

Perhaps it was. The outcome elicited memories of that day when Federer pulled off a Tsitsipas-like upset.

This summer, it will be 18 years since Pete Sampras' era of invincibility at Wimbledon was brought crashing down by a teenage Federer. Out with the old, in with the next-gen.

Tsitsipas showed no fear Sunday. He stood up to the two-time defending champion and beat him at his own game. Serving beautifully, Tsitsipas saved all 12 break points he faced. Even during the most tense moments, the Greek sensation looked like he was on the practice court, showing no fear.

Stefanos Tsitsipas has now won his first career matches against Roger Federer and Novak Djokovic. DAVID GRAY/AFP/Getty Images

The similarities between Federer and Tsitsipas are uncanny. Until this year, Tsitsipas had never won a match at the Australian Open, just like Federer hadn't at Wimbledon until his 2001 run. Both Tsitsipas, 20, and Federer, who was 19 at the time, were ranked 15th in the world and both had won just one title coming into the major. More parallels? Both matches came in the fourth round.

Sampras, who was still 29 at the time, retired a year later, right after winning the 2002 US Open. At 37, Federer knows his career has a shrinking shelf life, but he has won three Grand slam titles in the past two years to bring his total to 20, and he is much closer to his peak form than Sampras was in 2001.

Afterward, Federer, who fell in the opening week in Melbourne for only the second time in 15 years, acknowledged the comparisons between himself and Tsitsipas.

With his loss at the Australian Open, Roger Federer is projected to fall out of the top five. JEWEL SAMAD/AFP/Getty Images

"He has a one-handed backhand, and I used to have long hair too," Federer said.

But about that changing of the guard? Hyperbole?

"I love John [McEnroe]," Federer said. "I've heard that story the past 10 years. From that standpoint, nothing new there.

"About Stefanos, I think he's definitely done a really nice job now the last year and a half. I mean before that, too, obviously. But beating Novak [Djokovic] in Toronto, the likes of [Kevin] Anderson and [Alexander] Zverev, now me here. That's what you need to do to get to the next level. He's doing that. It's really nice for him."

Federer's ranking will fall from No. 3 to at least No. 6 in the world.

play 0:27 Tsitsipas: 'I'm the happiest man on Earth right now' Stefanos Tsitsipas reacts to defeating Roger Federer in the fourth round of the Australian Open.

Right from the start of the match Sunday, Tsitsipas looked as if he believed: in himself and that he could take down an all-time great.

Tsitsipas served superbly, especially when pressed to save those dozen break points. He never flinched, trusting in his ability to play the right shot at the right time.

Serena Williams' coach, Patrick Mouratoglou, whose academy is the training base for Tsitsipas, was courtside to watch the man he first spotted on YouTube playing in the prestigious Orange Bowl junior event.

In an interview with ESPN in September, he said there was "no limit" to what Tsitsipas could achieve, especially with his confident attitude and fighting spirit.

As for Federer, he didn't play badly. He just didn't convert his chances. The opening set was high-quality tennis, with Federer taking the tiebreaker after saving three set points.

That early setback could have taken the air from Tsitsipas, but anyone who has watched him over the past year knows he loves a battle. And so he continued to attack, mixing big serves, powerful forehands and dipping backhands. He attacked the net to keep Federer guessing.

In an era when so many players are pinned to the baseline, Tsitsipas looked to come forward whenever he could. He won 48 of 68 net points, and many of them were at crucial times. Federer, too, was aggressive with net play, winning 50 of 66 points.

But it wasn't enough. Federer suffered his second straight loss before the quarterfinals of a Grand Slam, marking only the second time he has done that since he won his first major, at Wimbledon in 2003.

"I have massive regrets tonight," Federer said. "I might not look the part, but I am. I felt like I have to win the second set. I don't care how I do it, but I have to do it. Cost me the game tonight."

Tsitsipas is on to his first Grand Slam quarterfinal, where Roberto Bautista Agut, who beat Marin Cilic in five sets Sunday, awaits.

Asked this week whether Tsitsipas was ready to make a deep run, Mouratoglou said he was "ready for Week 2" and that anything was possible.

That, for the record, is anything but hyperbole.