Katie Boulter's Great Britain defeated Argentina 2-1 in a huge boost to Australia's quarter-final hopes on a thrilling day four of United Cup action.
Boulter and Charles Broom sealed victory for Team GB with a 7-6 (7-4) 7-5 triumph over Maria Carle and Tomas Etcheverry in a one-hour, 45-minute deciding match at Sydney's Ken Rosewall Arena on Monday.
Etcheverry had squared the ledger with a 6-3 3-6 2-6 comeback win over Billy Harris after Boulter won the women's singles 6-2 6-3 against Nadia Podoroska.
Australia, after suffering a 2-1 loss to Argentina in their opening tie, can progress to the quarter-finals if they beat Great Britain on New Year's Day.
At Perth's RAC Arena, Kazakhstan locked in a spot in the quarter-finals after defeating Greece in both singles matches.
Former Australian Open runner-up Stefanos Tsitsipas fell first, beaten 6-4 7-6 (7-0) by Alexander Shevchenko.
Teams across Sydney and Perth who don't finish at the top of their group can also advance if they are the best runners-up out of the three groups in either city.
Boulter gave Great Britain the ideal start with her easy win over Podoroska in 86 minutes, seemingly enjoying a home-crowd advantage as fiance Alex de Minaur watched in the stands.
She announced her engagement to the Australian No.1 on Christmas eve after almost four years of publicly dating.
"We did them (Australia) a favour today giving them a 2-1 win so they can thank us later for that," Boulter said.
"We've got everything to play for. I'm going to come in all blazing."
The 28-year-old is unlikely to come face-to-face with de Minaur in the mixed doubles on Wednesday, with Matt Ebden and Ellen Perez the probable picks for Australia.
"I don't know about having a little match against him," Boulter said.
"I think that's going to be terrible and a bit of a popcorn match."
It is the second year in a row that Boulter, England's top women's player, and de Minaur have clashed on rival teams in the United Cup.
In Perth, Tsitsipas suffered defeat to a plucky Shevchenko after making 40 unforced errors to 17.
Greece were consigned to more misery, with Maria Sakkari losing 6-4 6-3 to Elena Ryabkina in the women's singles.
In the first of the two Monday night sessions, embattled world No.2 Iga Swiatek brushed off her doping controversy to lead Poland to a 2-1 win over Norway in Sydney. Â
Swiatek trounced Malene Helgo 6-1 6-0 before teaming with Jan Zielinski to edge out Casper Ruud and Ulrikke Eikeri 6-3 0-6 10-8 in a match tiebreaker in the deciding mixed doubles rubber.Â
Ruud had levelled the tie with a 7-5 6-3 win over Hubert Hurkacz in the men's match but Norway's defeat ended the Scandinavians' quarter-final hopes after they also lost their opening tie against Czechia.
Holders Germany beat China 2-1 in Perth, with both sides progressing to the quarter-final stage.
Laura Siegemund and Alexander Zverev defeated Zhang Shuai and Zhang Zhizhen 6-2 7-6 (7-3) in the deciding mixed doubles rubber.
Zverev, the world No.3, Â had started the tie with a 2-6 6-0 6-2 comeback victory over Zhang Zhizhen, which put Germany into the quarters, regardless of the eventual outcome.
China clinched their last eight spot when world No.168 Gao Xinyu upset Siegemund 6-1 3-6 6-3.
Germany advance as one of six group winners, and China as the best runner-up from the three groups in Perth. Germany and China had beaten Brazil 3-0 in their opening ties.
