The mixed doubles duo of Iga Świątek and Jan Zieliński earned the decisive point for Team Poland, overpowering Norway’s Casper Ruud and Ulrikke Eikeri 6-3, 0-6, 10-8 in a thrilling encounter, Polish state news agency PAP reported.
"It's still the old year, so I'm happy to finish it like that," Świątek, the world number two in women's tennis, said after the match.
She added: "I'm really happy that I could play with Jan and show our game. It was pretty tight, but I'm happy that at the end we stepped up the level and we finished it so Poland could win."
Earlier in the day, Świątek beat Norway's Malene Helgø 6-1, 6-0 in the women's singles, while her teammate Hubert Hurkacz, who is ranked 16th in the world, lost 5-7, 3-6 to Norway's Ruud in the men's singles.
Poland will play the Czech Republic on Wednesday for a place in the quarterfinals of the mixed-teams competition in Sydney.
Norway, meanwhile, has been eliminated from the United Cup.
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Source: IAR, PAP, atptour.com