Young Stefanos Tsitsipas signalled a changing of the guard at the top of world tennis as he shocked defending champion Roger Federer in the Australian Open fourth round Sunday.

The NextGen Finals champion beat Federer, who is 17 years his senior, 6-7 (11-13), 7-6 (7/3), 7-5, 7-6 (7/5) on Rod Laver Arena.

Tsitsipas, seeded 14, became the first Greek in history to reach the quarter-finals of a Slam where he will meet Spain’s 22nd seed Roberto Bautista Agut.

“There’s nothing really I can say to describe this,” said the 20-year-old. “I’m the happiest man on earth right now.”

The pair’s only previous meeting in the mixed team Hopman Cup earlier this month was a tight affair where Federer had won courtesy of two tiebreaks.

So there was little surprise when the first set also went to 6-6 and a decider.

It ended on a controversial note when a fan jumped the gun and shouted “out” on a Tsitsipas forehand with Federer 12-11 ahead.

The call distracted Tsitsipas, who mis-hit his next forehand and Federer put it away.

The second saw Federer constantly pressure the Greek’s serve but each time the energetic Tsitsipas resisted.

By the time the second set reached the tiebreak the gutsy Greek had already repelled 10 break points.

This time, however, Tsitsipas quickly took two points against Federer’s serve and levelled the match after just over two hours.

At 4-5 in the third, Tsitsipas forced his first two break points of the entire match but Federer weathered that storm.

But the clouds were darkening over the 20-time Grand Slam champion who was making an uncharacteristic series of mistakes, largely on his forehand wing.

On Federer’s next service game the youngster again got to 15-40, this time for set points, and a 42nd unforced error into the net saw the veteran behind in a match for the first time in the championship.

Tsitsipas called the trainer in the changeover after the seventh game of the fourth set a he began to cramp on a warm and sticky night.

Federer couldn’t take advantage of his opponent’s plight and Tsitsipas sealed the win on his first match point in the third tiebreak of the match.

“Roger is a legend of our sport,” said Tsitsipas. “I have so much respect for him. It’s a dream come rue.”