It was the question that eluded tennis players for over a decade.
How do you beat Pete Sampras?
A young Roger Federer faced the American powerhouse at Wimbledon in 2001, when Sampras was a four-time defending champion, and had won seven of the last eight.
Fresh-faced, armed with a dusty haircut and a one-handed backhand, Federer found a way to win. Fourth round, defending champ gone.
Now, there is the question that has eluded tennis players for nearly 20 years.
How do you beat Roger Federer?
Only a select few have managed to do so, and when they do, they know they've played some great tennis.
Stefanos Tsitsipas had a good look at Federer three weeks ago when they clashed in Perth at the Hopman Cup. Federer won in straight sets, but Tsitsipas had gained some crucial intel.
On centre court on Sunday night, they were inseparable. Tsitsipas had done his homework. Three of the four sets went to tiebreaks, the other a 7-5 result.
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So easily it could have gone either way, and had Federer won, the headlines would have been predictable; Brave Tsitsipas goes down, Federer clings on over young gun. The list goes on.
Fresh-faced, armed with a dusty haircut and a one-handed backhand, Tsitsipas - like Federer had over Sampras in 2001 - found a way to win. Fourth round, defending champ gone. The Swiss champion, gassed after a marathon first set tiebreak, struggled to keep the Greek kid down.
After the match, the same vibes circulated around Rod Laver Arena like they had at SW19 nearly 18 years earlier - and Tsitsipas had an understanding, even if he was three years old at the time.
Changing of the guard? Perhaps. Federer didn't think so after Sunday's match. But the similarities to the 2001 Federer-Sampras match were all too familiar.
"I haven’t watched that match here in Melbourne yet. Actually, that was fourth round, as well. That’s crazy. What a coincidence," Tsitsipas told reporters.
"I did watch that match when I was playing Wimbledon, grass courts. I did watch couple of his matches in the past that he played on hard courts.
"Yeah, I kind of watch some highlights of players that beat him in grand slams. It was actually not Rafa, Djokovic, but some other players that have beaten him in the past US Open. Very similar game style like me. I will not say which player. He did beat him in US Open a few years ago.
“It was a good example to see the way he played, the way he fought into the match to get that win. I’m pretty sure that helped today."
Tsitsipas was speechless speaking to John McEnroe after the match, saying he was the "happiest man on earth".
While the RLA crowd normally embraces the box office atmosphere Federer provides, the Greek flags soared around the arena and outside, the blue and white flying for their new hero.
Having had time to gather his thoughts and reflect on the result, Tsitsipas had to contend with the fact he had taken down his own hero.
It nearly didn't come to fruition. As a teenager, while swimming in Crete while there for an ITF Futures tournament, Tsitsipas almost drowned.
His inner strength spilled out into pure emotion as he acknowledged the crowd, pointing to himself. Did I just beat Roger Federer?
"It was a very emotional moment. It was something, a beginning of something really big. I felt joy. I felt happiness. I felt a huge relief going out of my shoulders," the 20-year-old said.
"That moment is definitely something that I will never, ever, ever forget. This match point is going to stay, I’m pretty much sure, forever, for the rest of my life.
"I just managed to close that match and stay strong, beat my idol. My idol today became pretty much my rival."
