Stan Wawrinka lost in three sets in gruelling Sydney heat of more than 40°C but Switzerland beat Belgium on Saturday to reach their first United Cup final.
Switzerland won 2-1, with Belinda Bencic playing a starring role, and will face either holders the United States or Poland in Sunday's decider.
The Tokyo Olympic champion Bencic put Switzerland on their way in the semifinals with a battling 6-3, 4-6, 7-6 (7/0) victory over Elise Mertens in two hours and 37 minutes.
"It feels like 170 kilos fell off my shoulders – I was so stressed, I really wanted to do well and today I felt so much pressure to not let my team down," Bencic said.
Then it was over to the 40-year-old Wawrinka, a three-time Grand Slam champion who was on Friday handed a wildcard to the Australian Open starting on 18 January.
Wawrinka, playing his final year before retirement, fought back to force a deciding set against Zizou Bergs, but the Belgian ultimately won 6-3, 6-7 (4/7), 6-3 in two-and-a-half hours in the searing heat.
That meant the semifinal went down to the mixed doubles, with Bencic and Jakub Paul defeating Bergs and Mertens in another three-set battle.
"The team spirit is amazing and it starts with the captain," Bencic said, referring to Wawrinka.
"He's supporting all throughout my match and then he goes out to play his own match and then he's back to support the doubles.
"It starts with him but then with all the energy from our bench it gives us such a spirit to win."
Poland and the United States play later Saturday.