Alex de Minaur's Team Australia face a perilous path to a maiden United Cup final after super powers Poland and the USA showcased their credentials ahead of the knockout stages in Sydney.
Poland set up a blockbuster quarter-final with Australia after Europe's two-time runners-up completed the last-eight line-up with a crushing 3-0 win over the Netherlands on Wednesday.
Big-serving Hubert Hurkacz sealed Poland's passage to a Friday night showdown against the host nation with a 6-3 7-6 (7-4) win over Tallon Griekspoor at Ken Rosewall Arena.
After routing Germany 3-0 in their opening tie, the Poles only needed one win against the Dutch to qualify for the quarter-finals.
But the ninth seeds fired a warning to their rivals with another statement 3-0 triumph to be the only team to advance with a perfect 6-0 match record from their two group encounters.
Her team's progression assured with two rubbers to spare, reigning Wimbledon champion and world No.2 Iga Swiatek followed up Hurkacz's victory with a 6-3 6-2 win over Suzan Lamens.
Katarzyna Kawa then combined with Jan Zielinski to complete Poland's second 3-0 sweep with a 6-3 3-6 [10-4] mixed doubles win over Demi Schuurs and David Pel.
Australia or Poland will progress to a semi-final against the top-seeded US after the defending champions edged out Greece 2-1 in a thrilling first quarter-final in Perth.
After Coco Gauff steamrolled Maria Sakkari 6-3 6-2 to put Team USA on the front foot, former Australian Open runner-up Stefanos Tsitsipas upset world No.9 Taylor Fritz 6-4 7-5 to send the tie to a deciding mixed doubles.
Gauff returned to the RAC Arena alongside Christian Harrison to deny Sakkari and Tsitsipas 4-6 6-4 [10-8] in a tense match tiebreaker to confirm the Americans' return to Sydney for the sudden-death stages.
The US almost blew a 5-0 lead and two match points in the breaker before prevailing.
"We got off to a lead and then, yeah, I thought it was a great level throughout," Gauff said.
"We just played free and played aggressive, but they were a great team. It wasn't easy, so I'm really happy that we were able to move through."
With five-time grand slam champ Swiatek a hot favourite to beat a somewhat ailing Maya Joint in the women's singles, de Minaur looms as the key to Australia's chances of upsetting the Poles.
But with a victory over world No.3 and 2025 AO runner-up Alexander Zverev also this week, the in-form Hurkacz has hit the ground running after missing almost seven months last year following arthroscopic surgery on his right knee.
Hurkacz fired 21 aces to beat Griekspoor in one hour and 24 minutes to book his date with world No.6 de Minaur.
"I didn't know what to expect from myself. You expect a lot, but then the reality might be different," Hurkacz said of his return after injury.
"I'm ready, but I still need time to get better and to develop things and to get matches.
"Alex has been very successful in the past years, and he's probably the highest career rank that he's ever had.
"It's going to be a good challenge, especially with the Aussie crowd. It's going to be a lot of fun, and I'm excited for the match."
In the second quarter-final at Perth's RAC Arena on Wednesday night, Switzerland advanced to the semi-finals with a 2-1 win over Argentina.
Belinda Bencic got Switzerland off to a hot start with a 6-2 6-2 demolition of Solana Sierra before Sebastian Baez levelled the tie with a 7-5 6-4 triumph over retiring veteran Stan Wawrinka.
Bencic and Jakub Paul won the deciding mixed doubles 6-3 6-3 against Maria Lourdes Carle and Guido Andreozzi, setting up a semi-final showdown for Switzerland against either Belgium or Czechia.
