World No.25 Tomas Machac has quit mid-match amidst an emotional on-court meltdown to hand the US free passage into Sunday's United Cup final against Poland.
The top-seeded US went 1-0 up in Saturday night's semi-final tie against Czechia at Ken Rosewall Arena after world No.3 Coco Gauff crushed Karolina Muchova 6-1 6-4.
Czechia was on track to level the tie after Machac took the first set against Taylor Fritz 7-6 (7-4) before racing out to a 5-2 lead in the second.
Machac couldn't convert either of his two match points on Fritz's service game, and the American broke back to get things back on serve.
Tensions were clear to see at the changeover as Machac berated himself and his team captain Daniel Vacek.
Machac threatened to throw a bottle on the floor during his initial meltdown before composing himself.
Vacek was also feeling the heat, throwing a towel over a camera focusing on him when Machac was back on court.
Fritz won the next two games to ensure an even feistier time at what proved to be the final changeover.
Machac threw his racket down where the rest of his team were sitting, with the racket seemingly grazing some of his team members as it bounced away.
He then embarked on an extended verbal tirade as Vacek watched on helplessly in front of him.
Machac's frustrations exploded to the point where he whacked his bag three times before he finally composed himself enough to return to the court.
He played just one more point before calling it quits, despite having no obvious physical ailment.
"It was a crazy match. It's not the way I want to win," Fritz said.
"He said he'd been cramping for a couple of games already. I didn't notice. He was moving pretty well. But I guess he was playing through it, fighting.
"It's really humid in here. That's something I was thinking of - if I could win that set and take it to a third, I thought someone might start cramping later on."
Poland secured their spot in the final with a 3-0 clean sweep of Kazakhstan.
Hubert Hurkacz gave Poland a 1-0 lead after defeating Alexander Shevchenko 6-3 6-2 in just 57 minutes.
Iga Swiatek had to fight much harder in her singles match against 2022 Wimbledon champion Elena Rybakina, but she was able to come from a break down in the opening set to post the 7-6 (7-5) 6-4 victory.
The five-time major winner had her upper right leg strapped on Saturday after appearing to tweak her thigh during her three-set quarter-final singles win over Katie Boulter two days earlier.
Swiatek was given the all clear to feature in the semi-final, and she overcame a sluggish start to secure an impressive win.
"On my last match it hurt me for some time," Swiatek said of the injury scare.
"And yesterday (Friday) I thought I was more panicking because it's hard to say after one day what's really going on there.
"But nothing happened in everybody's opinion. I'm ready to play some more matches."
The US got off to a hot start in their semi-final thanks to Gauff, who converted four of her eight break-point opportunities to seal the win in just 86 minutes.
Gauff spent the group stage and quarter-finals in the mixed-teams tournament in Perth, but the trip to Sydney and the three-hour time difference didn't slow her down on court.
"I woke up so late today," Gauff said.
"The whole match I was intense and focused, so I think that made the difference today."
