Australia put one foot in the Davis Cup final after Alexei Popyrin beat Finland's Otto Virtanen in the first singles rubber on Friday for a 1-0 lead.
World number 40 Popyrin dispatched Virtanen 7-6 (7/5), 6-2 to give last year's runners-up control of the tie.
The Finn, ranked 171st, excelled in the quarter-final win over reigning champions Canada but after a close first set was comfortably beaten.
Finnish fans took over the Martin Carpena arena again as the team played in a Davis Cup semi-final for the first time.
Virtanen was putting more pressure on Popyrin's serve than vice-versa and forced set point at 6-5 up.
The 22-year-old could not capitalise, sending a return long and Popyrin held after two deuces for a tie-break.
The duo exchanged mini-breaks before the Australian triumphed when Virtanen overhit a forehand.
In the second set Popyrin's superior quality told with Virtanen double-faulting to hand his opponent a break for 4-2, and then going wide as the Australian broke again to seal the win.
"It was an amazing feeling, one of the highlights of my career, unbelievable," said Popyrin, after his first victory at a Davis Cup finals.
Finland's number one Emil Ruusuvuori, 69th in the world, faces Australia's Alex de Minaur in the second singles rubber needing to win to keep his country's fairytale run going.
Ruusuvuori missed the win over Canada with a shoulder problem, while De Minaur produced a stunning comeback from a set and a break down to help Australia past the Czech Republic.
On Saturday Novak Djokovic's Serbia face Jannik Sinner and Italy in the second semi-final.
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