Stefanos Tsitsipas needed a match tiebreak to see off Taylor Fritz and restore Europe's lead in the Laver Cup.
Tsitsipas started the match at a canter, breaking in the first game of the first set, which he claimed 6-2.
However, a slow start to the second allowed Fritz to level it with a 6-1 set, forcing a match tiebreak, which the Greek eventually edged.
"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.
"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."