World No.19 Frances Tiafoe sealed the win for the US with a tough 3-6 7-5 6-3 triumph over Britain's Dan Evans on Wednesday night, giving his country an unassailable 3-1 lead in the tie.
Madison Keys had earlier halted British upstart Katie Swan's fast start, winning 2-6 6-3 6-4, before Cameron Norrie levelled the tie with a 6-4 5-7 6-4 win over Taylor Fritz.
Jessica Pegula cruised past Harriet Dart to give the US a 2-1 lead, before Tiafoe saved a series of clutch break points in the second set on the way to beating Evans.
"We have our battles.Every time we play, we go to war," Tiafoe said of his thrilling match against the world No.27.
"I'm happy the USA can keep going. It feels good. I did want to beat GB. I love all of those guys. It was a great battle today."
In Perth, Tsistipas avoided a nasty case of deja vu to level Greece's city final tie with Croatia at 1-1.
The favourite was in complete control before Borna Coric launched a brave fightback, saving two match points in the second set and five in total before finally falling 6-0 6-7 (4-7) 7-5 .
Coric had saved six match points on his way to beating Tsitsipas at the 2020 US Open and looked on track for a repeat when he twice had breaks in the deciding set.
Tsitsipas, mistake-free in the first set, cast aside the disappointment of three more missed match points at 6-5 to finally take the crucial point for Greece.
"I just didn't want to lose," he said.
"I said to myself, 'No, no, you're not getting off this court with your head down, fight through it'.
"I had to go 110 per cent today."
Coric's teammate Donna Vekic had earlier defeated Greece's Despina Papamichail 6-2 6-0.
Italy lead Poland 2-1 in the Brisbane city final.
Lorenzo Musetti flogged Daniel Michalski 6-1 6-1 in the opening match to give Italy a 1-0 lead, before Poland's world No.1 Iga Swiatek levelled it up with a 6-2 6-4 win over Martina Trevisan.
Italy's lead was restored when world No.16 Matteo Berrettini posted a tough 6-4 3-6 6-3 win over world No.10 Hubert Hurkacz.
The winners of the three city finals, plus the best-performing loser, will contest the semi-finals of the new mixed teams event in Sydney from Friday.
17°C