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

Merely getting the chance to play against Roger Federer at a Grand Slam tournament is a thrill for many a young tennis player. Beating Federer? Well, that's a whole other story.

Federer's bid for a third consecutive Australian Open title ended in a surprising 6-7 (11), 7-6 (3), 7-5, 7-6 (5) loss Sunday night to 20-year-old Stefanos Tsitsipas, the first player from Greece to reach a major quarterfinal.

"Roger is a legend of our sport. So much respect for him. He showed such good tennis over the years. I've been idolizing him since the age of 6," Tsitsipas said. "It was a dream come true for me ... just facing him. Winning at the end? I cannot describe it, you know."

The key stat Sunday night: Federer failed to convert any of 12 break point opportunities. Tsitsipas went 1-for-3 in that category, coming through in the last game of the third set.

"I have massive regrets," Federer said.

At 37, Federer was the oldest man left in the field and was bidding to become the oldest man to reach the quarterfinals in Australia since Ken Rosewall, who was 43, in 1977.

Tsitsipas, a lanky guy who kept his scraggly hair in place with a pink headband, was the youngest man in the fourth round this year.

He lost his opening match a year ago, when Federer picked up his sixth Australian Open championship and 20th Grand Slam title overall.

At least Federer was able to crack a joke when asked whether Tsitsipas reminds him of a younger version of himself, replying: "He has a one-handed backhand. And I used to have long hair, too."

This one was a thriller from beginning to end, both in terms of the high quality and entertaining style of play from both men -- something long expected of Federer. The world is still learning what the 14th-seeded Tsitsipas can do. His soft hands serve him well on volleys, and he's that rare man who will press forward as often as Federer does and will have nearly as much success.

On this cool evening, Tsitsipas won the point on 48 of 68 trips to the net, while Federer went 50-for-66.

Tsitsipas served well, too, compiling a 20-12 edge in aces and, more significantly, staving off all of those break chances that Federer earned: two in the first set, eight in the second and two in the third.

And as is often the case when a youngster outdoes an old master, there was a buzz about whether this might signal something more meaningful.

"I see him being high up in the game," Federer said, "for a long time."

Tsitsipas will next play Roberto Bautista Agut, who beat 2018 runner-up Marin Cilic in five sets to reach the quarterfinals for the first time. Bautista Agut had previously lost in the fourth round nine times at Grand Slams.

Second-seeded Rafael Nadal swept to his 20th victory in 24 attempts over Tomas Berdych with a 6-0, 6-1, 7-6 (4) win to advance to the quarterfinals.

Nadal won the first nine games of the match. When Berdych finally got on the board in the 10th, he held up his left arm up in mock celebration. Berdych came back strongly in the third set and had a set point in the 12th game before Nadal dominated the tiebreaker.

The last time the players met here in 2015, Berdych beat the Spaniard in straight sets to end a 17-match losing streak against Nadal.

It marks the 11th time Nadal has reached the quarterfinals here. He will next play American Frances Tiafoe.

"When you're back, you need a little bit of the luck in the beginning," said Nadal, who didn't play a competitive match between last year's US Open and the season-opening major in Australia because of injuries. "I'm in the quarterfinals, let's see what happens now."

Tiafoe celebrated his 21st birthday in style with a 7-5, 7-6 (6), 6-7 (1), 7-5 win over 20th-seeded Grigor Dimitrov to advance to his first Grand Slam quarterfinal.

Tiafoe, who received treatment on his right forearm at the end of the third set, broke Dimitrov's serve in the 11th game of the fourth, then held serve to end the 3-hour, 39-minute match.

In typical style, Tiafoe took off his T-shirt, flexed his right bicep and waved to the crowd. Then he sat in his chair and covered his head with a towel.

It's a big turnaround for the American at the Australian Open. He lost all three singles matches at the exhibition Hopman Cup three weeks ago, then was beaten in the first round at the Sydney International.

At Melbourne Park, he has beaten No. 5 Kevin Anderson and veteran Andreas Seppi.

Information from The Associated Press was used in this report.