An inspired Viktoria Hruncakova defied her lowly ranking to defeat Kim Birrell in a tense three-setter before in-form Rebecca Sramkova sealed victory with a confident win over injured veteran Ajla Tomljanovic.
Australia had been chasing a fourth semi-final appearance in five years, and a first title in half a century in the prestigious teams' event formerly known as Fed Cup.
Instead, Sam Stosur's first year as captain ended in disappointment at Marten Carpena Arena as the Slovakian underdogs continued their dream week in Spain.
Stosur pulled somewhat of a selection surprise in calling up Birrell ahead of Olivia Gadecki for the opening singles rubber.
While both options had made maiden WTA Tour finals in recent times, Gadecki was higher-ranked after breaking into the world's top 100 for the first time following a charge to the Guadalajara Open title decider in September. Birrell, though, was also riding high after making last month's Japan Open final and entered Sunday night's match well-fancied to account for the 241st-ranked ranked Hruncakova.
Alas, the 26-year-old world No.115 went down 7-5 6-7 (4-7) 6-3 in a rollercoaster two-hour, 31-minute battle to pile the pressure on the resurgent Tomljanovic.
Back as Australian No.1 after landing a second career title in Hong Kong Kong following more than a year out with a knee injury, Tomljanovic's return to the competition ended in despair.
The 31-year-old needed a medical time-out in the first set for another left knee issue and never recovered, going down 6-1 6-2 in barely an hour.
Sramkova had upset world No.11 Danielle Collins, the 2022 Australian Open runner-up, in Slovakia's shock win over the USA on Friday to make the quarter-finals.
Now the 28-year-old looms as pivotal to Slovakia's chances of continuing their giantkilling run against either defending champions Canada or Great Britain in the semi-finals.
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