Alex de Minaur notched the only victory against Argentina, the world No. 9 putting on a devastating display to beat Tomas Etcheverry 6-1 6-4 at Sydney's Ken Rosewall Arena in 75 minutes.
His win squared the tie after Nadia Podoroska beat Olivia Gadecki 6-2 6-4 in Saturday's women's singles earlier.
Olympic doubles gold medallist Matthew Ebden and Ellen Perez had to finish off the job in the mixed doubles against Etcheverry and Maria Carle - who came in for Podoroska - but instead lost 6-2 6-4.
Australian captain Lleyton Hewitt is taking little out of their defeat, having also endured a first-round loss last year before scraping their way to the quarter-finals.
His team returns to action on New Year's Day when they face a Great Britain outfit including Billy Harris and de Minaur's fiancee Katie Boulter.
"We'll give ourselves every chance," Hewitt said.
Cheered on by a boisterous home crowd in the second, the 22-year-old looked ready to mount a comeback after going ahead 4-3 but was ultimately undone by her errors in the 88-minute contest.
"All we can do is prepare as well as possible. We've got three days off now as well so it's really just trying to prepare.
"No matter what happens though, it's great preparation for these guys going into the Australian Open."
All smiles from Lleyton Hewitt 😃🇦🇺— United Cup (@UnitedCupTennis) @alexdeminaur is going deep into his bag of tricks at the #UnitedCup pic.twitter.com/fEFnGdJArlDecember 28, 2024
In his singles match, Sydney local de Minaur claimed the first set against Etcheverry within 28 minutes and sent the near-capacity crowd rocking with every point earned.
Etcheverry took four hard-earned games in the second set after finding his rhythm, but de Minaur prevailed with great precision.
"Alex, I thought, played fantastic tennis tonight," Hewitt said.
"He was very confident out there, right from the word go, played his Plan A out there, and executed incredibly well.
"He played a faultless match, really."
Olivia Gadecki getting the Aussie crowd on their feet 🇦🇺— United Cup (@UnitedCupTennis) #UnitedCup pic.twitter.com/7y4yg9Fwc9December 28, 2024
In the women's singles, emerging star Gadecki battled nerves and was broken in her opening game, before losing the first set.
Cheered on by a boisterous home crowd in the second, the 22-year-old looked ready to mount a comeback after launching ahead 4-3 but was ultimately undone by her errors in the 88-minute contest.
"I thought Olivia got better as the match went on," Hewitt said.
"If we could have snuck out the second set and taken it to a deciding, I think she would have got better throughout the third set."
The scoreline was the same as when Gadecki last met Podoroska at this year's Indian Wells Open.
Gadecki is the nation's only woman to gain direct entry to the Australian Open, having reached the world's top 100 with a career-best run to the Guadalajara Open final in September.
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