Hubert Hurkacz stunned world number nine Taylor Fritz in the men's singles to give Poland the lead, before Coco Gauff beat Iga Swiatek to level the tie. But Poland prevailed in the mixed doubles decider, where Katarzyna Kawa and Jan Zielinski beat Gauff and Christian Harrison.
Hurkacz, ranked 83rd in the world, avenged his loss to Fritz in last year's final in a 7-6(1), 7-6(2) win, after taking the lead in both tiebreaks with well-placed backhand passing shots to beat his second top-10-ranked opponent this week.
"Definitely pleasantly surprised... in seven months, (this is) my first tournament. So coming back from such a long period the first time in my life, you never know what to expect," said Hurkacz, who also beat world number three Alexander Zverev on Monday when he returned from an injury layoff.
World number four Gauff got the US back in the game, beating Swiatek 6-4, 6-2, as the American's powerful shots forced the world number two to stay near the baseline and make mistakes with her backhand returns.
Gauff, however, could not complete her team's comeback, as Kawa and Zielinski won 7-6(5), 7-6(3) in the mixed doubles, ensuring Poland's progress to Sunday's final.
"I'm so proud of our team... I gave everything that I had. It means a lot, it's a great victory for us," Kawa said.