And it was none other than Alex de Minaur's fiancee Katie Boulter, who twisted the dagger with a merciless 6-2 6-1 victory over Olivia Gadecki on Wednesday night.
Australia entered the tie needing to win 3-0 to top Group F and secure a path to the quarter-finals.
Instead Team GB automatically advance to a last-eight showdown on Friday night in Sydney with Italy.
A 2-1 triumph could still be enough for the tournament hosts to progress as the best runners-up out of Sydney, but only if Australia win the men's singles and mixed doubles both in straight sets and only drop a few games.
But a week after announcing her Christmas-time engagement to de Minaur, Boulter put business before pleasure to place her beau in quite the predicament at Ken Rosewall Arena.
"Yeah, very happy to start my new year like that.
"I feel like I've actually lost a lot of sleep recently over the thought of playing my fiancee in doubles," Boulter said.
"So I'm so tired and just very happy to get through this rubber. I'm not  going to lie.
"Wow, yeah, very happy to start my new year like that."
Katie Boulter embraces Lleyton Hewitt and admits she wants Australia to progress in the United Cup. (Mark Evans/AAP PHOTOS)
World No.6 de Minaur is a hot favourite to take care of Billy Harris to level the tie, but he will need to win handsomely to give Australia's mixed team a hope in any potential deciding rubber.
Boulter featured in the mixed in Great Britain's win over Argentina on Monday, but the Australian captain would need to relegate reigning Olympic doubles champion Matt Ebden for the two love birds to clash on Wednesday night.
Asked if she could help Australia get through, Boulter said: "There's actually nothing I would love more than for Australia to come second."
"So let's see what they can do.
"But I still want GB to win everything, so I'm selfish."
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