Tsitsipas, ranked 14th in Melbourne, shocked Roger Federer with a 6-7, 7-6, 7-5, 7-6 win.
The 20-year-old saved a remarkable 12 break points on his way to victory and will face Roberto Bautista Agut in the quarter-finals.
In a tense encounter three sets went to a tie-break as the serve dominated throughout.
But Tsitsipas, who had twice been penalised for a shot-clock delay in the very first game of the match, has attributed his win to a strong mental attitude after knocking out the 20-time Grand Slam winner.
“I was just serving well,” Tsitsipas said.
“Even sometimes I didn’t serve first serve, sometimes I would start the point with a second serve.
“I think I was just very aggressive from the very beginning from his return.
“I managed to not make any mistakes, stay in the point, make him play.
“Most of the times he missed. Most of the times I saved those break points.
“It was actually very mental, I would say. I could have cracked at any moment, but I didn’t because I really wanted it bad.
“I showed it on the court. Obviously and for sure that mental toughness helped a lot.
“It could have been a different match if I wouldn’t stand that pressure.”
It was the first encounter between Tsitsipas and Federer with a brilliant match ensuing.
And the Greek star admitted it was a dream to be the other side of the net to one of his heroes.
“There’s nothing I can describe it to,” Tsitsipas added. “I am the happiest man on earth.
“From the beginning I went in believing in myself and my abilities.
“Roger is a legend, he showed such a good tennis over the years, I’ve been analysing him since the age of six, and this is a dream to face him.
“Winning this – I cannot describe it.”