Poland will face the United States in Sunday's final of the mixed-team United Cup following a dramatic end to the semi-finals in Sydney.

Iga Swiatek beat Elena Rybakina in straight sets to seal her side's victory against Kazakhstan while the US won 2-0 against the Czech Republic.

Coco Gauff beat Karolina Muchova 6-1 6-4 in the women's singles before Czech player Tomas Machac retired when leading 7-6 (7-4) 5-6 against Taylor Fritz.

"He said he had been cramping for a couple of games - I didn't notice," said Fritz, who had broken back after facing two match points and was serving for the second set when Machac approached the net to concede.

"He was moving pretty well, but I guess he was playing through it and fighting it.

"It's really humid in here. That's something I was thinking about as a possibility - if I could win that set and take it to a third, he might start cramping later on. It's tough conditions and it was a physical match."

In the earlier semi-final, world number two Swiatek gave Poland an unassailable 2-0 lead with a 7-6 (7-5) 6-4 win over sixth-ranked Rybakina.

Hubert Hurkacz had earlier put Poland on their way with a 6-3 6-2 victory over Alexander Shevchenko in the men's singles.

"This win makes me really proud - it is perfect preparation for the Australian Open," said Swiatek, in reference to the year's first major that starts in Melbourne on 12 January.

Swiatek has never made it past the semi-finals of the Australian Open, although she has won the US Open on the same hardcourt surface, with her other four Grand Slam titles coming on clay at the French Open.

The 23-year-old Pole, who played with strapping on her right thigh, was taken to a first-set tie-break after Rybakina saved two set points before she pulled away.