Petra Kvitova upset world number three Jessica Pegula on Friday, but the United States still won their United Cup tie against the Czech Republic in Sydney.
Two-time Wimbledon champion Kvitova clinched a thriller, saving seven set points in the first set before winning 7-6 (8/6), 6-4 at the Ken Rosewall Arena.
After the Americans won the opening two rubbers on Thursday, Kvitova's victory gave her country hope in the inaugural tournament, which features two men's and two women's singles matches along with a mixed doubles.
But the US clinched the tie when Tomas Machac injured his ankle against 19th-ranked Francis Tiafoe and was forced to retire hurt while down 6-3, 2-4.
Kvitova, ranked 16, looked back to her best against Pegula, who burst into the top 10 in 2022 by winning the WTA 1000 tournament in Guadalajara for the biggest title of her career.
After a topsy-turvy first set that could have gone either way, she got a double break at the start of the second set and while the American got one back, Kvitova was able to hold on and take the match in two hours.
"It was a high-level match for a solid two hours, it got kind of physical towards the end," said Pegula.
"She can really step into the court, hit lots of winners and hits a big ball. It's not always easy playing someone like her first match of the season."
Tiafoe, a US Open semifinalist this year, started brilliantly against Machac, racing through the first set with relative ease.
Machac fought back in the second and was a break up before rolling his ankle and retiring.
"He picked up his game and broke me twice -- I was a little sloppy there," Tiafoe said.
The Czech Republic will face Alexander Zverev's Germany on Saturday while the US have two days off before they close out group play against the Germans.
Switzerland completed a 3-0 defeat of Kazakhstan in Brisbane, with captain Stan Wawrinka defeating Alexander Bublik 6-3, 7-6 (7/3) a day after wins from Olympic champion Belinda Bencic and Marc-Andrea Huesler.
'GREAT LEVEL'
Wawrinka, a three-time Grand Slam champion who has struggled with injury, finished his match with a winning forehand to the corner.
"It feels good to be back in Australia," the 37-year-old said. "It's been a long time since I played here.
"Tonight I had a great level," he added. "I'm super happy with how I'm playing. I had to play aggressive and push him at every opportunity."
Italy wrapped up their tie against Brazil when they won both singles matches in Brisbane, with the tournament being played across three cities.
Matteo Berrettini downed Thiago Monteiro in a gruelling clash 6-4, 7-6 (9/7) before Lucia Bronzetti saw off an outclassed Laura Pigossi 6-0, 6-2. Brazil face Norway on the weekend while Italy take on the Norwegians on Monday and Tuesday.
In Perth, world number six Maria Sakkari gave Greece an unassailable 3-0 lead over Bulgaria when she beat Viktoriya Tomova 6-3, 6-2.
"I was a little nervous in the beginning, especially being 2-0 up in the tie, I knew my match was crucial in winning the tie," said Sakkari. "She was very solid, but I found my game and got better."
Her teammates Stefanos Tsitsipas and Despina Papamichail won their matches on Thursday.