Roger Federer acknowledged he was beaten by the better player as he predicted a bright future for his Australian Open conqueror Stefanos Tsitsipas.

Twenty-year-old Greek Tsitsipas produced a wonderful performance inside Rod Laver Arena to beat last-16 opponent Federer in four tight sets, ending the defending champion's hopes of claiming a third straight title at Melbourne Park.

In a post-match news conference, 20-time grand slam winner Federer said:

"I lost to a better player who was playing very well tonight. [He] hung in there, gave himself chances at some point, stayed calm. It's not always easy, especially for younger guys. Credit to him for taking care of that."

Following an on-court interview with the victorious Tsitsipas, John McEnroe hailed the upset as a "changing of the guard" in men's tennis.

"He's in front of the mic a lot. He's always going to say stuff," said Federer. "I love John. I've heard that story the last 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."

"I see him definitely being high up in the game for a long time. That was a good night for him tonight."

Federer and Tsitsipas also met earlier this month at the Hopman Cup in Perth, with the Swiss winning two tie-breaks to claim victory.

Despite prevailing on that occasion, Federer has now failed to break the serve of Tsitsipas in six sets, after spurning 12 opportunities to do so on Sunday.

"There is always multiple factors that play into a match like this. But it definitely didn't go the way I was hoping on the break points," the third seed acknowledged.

"I also didn't break him at the Hopman Cup, so clearly something is wrong [with] how I return him, what I'm trying to do. He's doing a good job to defend them. Nevertheless, it's very frustrating."