The Greek star beat his fellow 21-year-old – and Laver Cup rookie – 6-2 1-6 10-7 in front of a capacity crowd at Palexpo, edging Europe into a 2-1 lead over Team World after three matches.
“If you would tell me 10 years ago that I would be playing an event having Roger and Rafa and four more top-10 guys by my side, I wouldn’t have believed you,” said Tsitsipas.
“That’s just magnificent.”
It’s going to be a long weekend with a lot of tight matches, but we’re happy that we’re up 2-1 – Bjorn Borg
Roger Federer and Alexander Zverev will take on Jack Sock and Denis Shapovalov in the final match of day one.
Tsistipas looked every inch the top-10 player in the opening set, racing through it in 24 minutes with two breaks, winning 93% of the points on his first serve.
While he headed to the changeover in purposeful fashion, the red bench gave their man a team pep talk – and it produced results.
The Californian, ranked No.30, played superbly to win six of the next seven games, breaking serve in game two and again for 5-1 as his forehand began to dominate.
One rasping winner had Fritz celebrating and pointing at his captain, with McEnroe leaping off the bench and responding in kind.
The famed Team World camaraderie looked to be helping Fritz take command but after Tsitsipas left the court for a meeting with his entire team, he returned in determined mood.
Tsitsipas saw a 5-2 lead disappear in the tiebreaker as break points dominated but kept his cool to capitalize when Fritz double-faulted at 7-7, firing a superb forehand to earn two match points and converting the first with a big serve.
“I broke my shoelaces at 2-2 and I had to continue,” said Tsitsipas.
“I didn’t want to stop the match. That was quite tough because I was scared I’m going to lose my shoe during the point.
“I think the momentum changed in the third set tiebreak. I found my rhythm, had a more clear mind on the court, didn’t rush so much, so I think that played a crucial role in closing it at the end.”
Europe Captain Bjorn Borg added: “It’s not easy for anyone to play first time Laver Cup, it’s a lot of pressure.
“They understand it’s very serious, a lot of pressure, a lot of prestige. It’s going to be a long weekend with a lot of tight matches, but we’re happy that we’re up 2-1.”