Alex de Minaur's Australian Open preparation hit an early snag with a decisive 6-3 6-3 loss to Britain's Cameron Norrie before Zoe Hives fell to Katie Swan, consigning Australia to a winless start at the United Cup.
The Australians did not take a set away from Great Britain on day one of the new mixed-gender team tournament and must win their mixed doubles clash and remaining two singles matches on Friday to claim the tie.
Nick Kyrgios' late withdrawal with an ankle injury meant de Minaur had less than 36 hours' notice he would be playing world No.14 Norrie at Ken Rosewall Arena on Thursday night.
The Sydney crowd made its feelings known; one spectator wore a Kyrgios tee shirt with a clown wig and makeup, while others held signs referencing Kyrgios' scratching.
Despite the last-minute change, world No. 24 de Minaur would've liked his chances of causing an upset, having defeated Norrie in the pair's only previous meeting this April.
But the Australian failed to convert three consecutive break points early in the first set in what proved to be his only chance to assert himself on the contest.
Norrie shut the local crowd out and clawed back to deny de Minaur the chance to break, winning that game and then scoring a break of his own in the next one.
"Alex had that early chance and wasn't able to take it," said Australian captain Lleyton Hewitt.
"But then Cam did very little wrong from start to finish. It just wasn't our night tonight."
De Minaur's serve packed a punch - his first of the night slammed into the net and brought the chair umpire down to conduct some emergency repairs.
But Norrie's precision from the baseline was unmatched and when the Brit won a rally to break de Minaur a second time and go up 3-2 in the second set, the Australian's chances were fading.
He came close to his own first break point of the night in the next game but could not convert on deuce as Norrie edged to victory in 91 minutes.
"I made a couple of errors in certain areas and just didn't play my best overall tennis," de Minaur said.
World No. 772 Hives is the lowest-ranked of the 72 singles players participating in the United Cup and only returned to tennis in January after more than two years out battling a heart condition.
The Australian did well to claw back from 4-1 down in the first set and claim two break points but was left to rue unforced errors against world No.145 Swan as she dropped the first set.
Undeterred, Hives claimed the first three games of the second set before Swan won the last six to claim a comfortable 6-4 6-3 win.
In Friday's singles matches, Australia's top women's seed Ajla Tomljanović meets Harriet Dart before Jason Kubler squares off against Daniel Evans.