"I'm speechless. I have no words to describe what just happened on the court. My tennis speaks for itself," Tsitsipas said after the 3-6, 2-6, 7-6 (4), 6-4, 7-5 win.
Second-seeded Nadal, 34, took the first two sets fairly easily to run his streak of consecutive sets won at major tournaments to 35, one shy of Roger Federer's record in the professional era.
But he made some mistakes in a third-set tiebreaker and dropped it.
Tsitsipas, 22, then took the fourth set, too.
"I don't know what happened after the third set. I flew like a little bird," Tsitsipas said.
Previously, Nadal had only once lost a Grand Slam match after taking the first two sets. That happened at the 2015 US Open against Fabio Fognini.
"I missed couple of balls in the tiebreak that I shouldn't, that I could not miss if I want to win," Nadal said. "And that's it. I think Stefanos played a great fourth and fifth set.
"I was there. I played a couple of bad games. Just well done for him."
Nadal and Federer are currently tied at 20 Grand Slam singles titles, more than any other man in the history of tennis. There won't be a chance at 21 this week.
"Australian Open is one of my favorite events, without a doubt," Nadal said. "So I missed an opportunity to be in that semifinals again. And that's it.
"Well done for him. He played better than me probably in important moments. Was an equal match."
In his post-match news conference, Tsitsipas said that there was no major mental adjustment that turned things around.
"The thing is that I wasn't really thinking about a lot of things," he said. "Nothing was going through my head. I was so much -- how would I describe myself? Nirvana. Just, like, there. Playing, not thinking."
Earlier Wednesday, here's how uncomfortably hot it was during Daniil Medvedev's quarterfinal victory over Andrey Rublev: The winner needed to get a massage for cramping in his left thigh before he could stand up for his on-court TV interview.
The No. 4-seeded Medvedev moved into the third Grand Slam semifinal of his career and pushed his current winning streak to 19 matches by beating his good pal Rublev 7-5, 6-3, 6-2.
Medvedev said he had a hard time moving his left leg on the last three points while serving out the victory, and he could tell Rublev was cramping, too. A couple of times during a crucial portion of the second set, Rublev went over to a courtside box meant to hold the playe
"It was not easy," Medvedev said.
The all-Russian matchup only lasted about two hours, but it was still a struggle, with so many lengthy baseline back-and-forths -- one particularly grueling point contained 44 strokes, including the serve -- and the mid-afternoon sun making the temperature above 80 degrees Fahrenheit.
"We had some unbelievable rallies, and after the point, it's tough to breathe," said Medvedev, the runner-up to Nadal at the 2019 US Open and also a semifinalist on New York's hard courts last year after beating Rublev in the quarters.
"Actually," Medvedev said with a wry smile, "I think I am one of the first players to make Andrey that tired on the court, so I'm quite happy about it."
On Friday night, Medvedev will face Tsitsipas for a spot in the final.
The other men's semifinal will be No. 1 Novak Djokovic against 114th-ranked Russian qualifier Aslan Karatsev. The 27-year-old Karatsev is playing in the main draw of a major tournament for the first time, and he's the only man in the professional era to make it to the semifinals of his Grand Slam debut.
He and Medvedev give Russia a pair of semifinalists at the same major for only the third time.
"Amazing," Medvedev said about Karatsev's run so far. "It's unbelievable."
Medvedev and Rublev have known each other since they were kids. Medvedev -- at 25, two years older -- has always gotten the better of their matchups as pros, taking every set they've contested, mixing styles and covering the court so well with all of his 6-foot-6 frame.
This time, it was even up until 5-all, when Medvedev nosed ahead with a hold, then broke at love to win the set with the help of a pair of missed backhands and a netted forehand from Rublev.
Another key stretch arrived midway through the second set.
With Medvedev up 3-2, and Rublev serving at deuce, they settled in for that 44-swing point that just had to take a physical toll on both men. And while Rublev earned that point when his friend-slash-foe missed a forehand, he seemed sapped afterward.
That's when Rublev first took a seat for a bit of a respite. Later, at a changeover, he slumped back and leaned his head down, as though taking a nap on a park bench.
He managed to hold to 3-all, then really could have grabbed control of that set when he earned a trio of break points in the next game. But Medvedev steeled himself, played some terrific defense, and held to begin a three-game, set-claiming run.
Rublev went to the locker room before the third, returning with wet hair, looking as if he had tried to cool off by hopping into the shower. It might have made him feel better, but did nothing to alter the outcome.
"My game matches his quite well. I've known him [for a] long time," Medvedev said, "so I know how to neutralize his amazing, big shots."
Information from The Associated Press was used in this report.