Stefanos Tsitsipas rallied from a set down to topple Italy's Matteo Berrettini and keep Greece in the hunt to reach the final of the inaugural United Cup, where the United States awaits the winner.
The world number four came through a do-or-die clash in Sydney 4-6, 7-6 (7/2), 6-4 to earn a crucial point for his country.
Italy went into day two of the semi-finals with a 2-0 advantage after a gutsy Martina Trevisan upset world number six Maria Sakkari 6-3, 6-7 (4/7), 7-5 in a gripping encounter on Friday.
Trevisan's teammate Lorenzo Musetti then crushed Greek teenager Stefanos Sakellaridis 6-1, 6-1.
With Tsitsipas winning, the pressure is now on Valentini Grammatikopoulou who plays Lucia Bronzetti in another must-win clash. If she does, the tie will go to a deciding mixed doubles rubber.
"I hung in there and was just waiting for my chances, he's a big server," said Tsitsipas. "I was able to play with a lot of intensity."
Berrettini, ranked 16, took a grip on the high-quality match with an unanswered break in game seven of the first set.
They went toe-to-toe in set two before the pumped-up Greek star prevailed in a tiebreaker to take it to a third set where he ground down his tiring opponent.
- Served well -
Earlier, a composed Taylor Fritz beat Poland's Hubert Hurkacz in a battle of top 10 players to power the United States into the final.
The world number nine was too good for his big-serving opponent, ranked one place behind him, winning 7-6 (7/5), 7-6 (7/5).
Fritz came into the match with the United States already 2-0 up after Jessica Pegula stunned world number one Iga Swiatek 6-2, 6-2 on Friday and Frances Tiafoe swept past Kacper Zuk 6-3, 6-3.
His victory gave them an insurmountable lead before Madison Keys made it 4-0 by beating Magda Linette 6-4, 6-2.
"I think the difference in the tiebreakers was I just served well and I put some returns in the court, played some really solid points and didn't give him anything," said Fritz.
"I'm super excited going into the finals and I think we have been the favourites all week," he said.
Fritz earned break points at 4-4 in set one after stepping up the pressure on Hurkacz's booming serve, but the lanky Pole saved them both and turned the tables to earn two set points at 5-4.
But he too was denied and it went to a tiebreak, where the American dug deep.
Nothing separated them in the second set, with Hurkacz slamming his racquet to the ground as his frustrations grew.
It again went to the tiebreak, where two unforced errors from the Pole gave Fritz match point and he made no mistake.
Keys kept her unbeaten record at the tournament intact with a battling display against Linette.
After grinding through the first set, she went a break down in the second but was ultimately too strong for the 48th-ranked Linette.
"I feel like I've been playing really well on these courts and to be able to have so many matches on them already and get as comfortable as possible definitely helped me today," said the world number 11.