Great Britain are one win away from their first Davis Cup final since 1978 after Andy Murray and brother Jamie overcame Australia's Lleyton Hewitt and Sam Groth in the doubles.
After Friday's singles matches had been split, the British pair triumphed in a frenetic four-hour, five-set encounter in Glasgow, triumphing 4-6 6-3 6-4 6-7 (6-8) 6-4.
Having produced a fine response to losing the first set and then coming back from a break down to win the third set, Great Britain looked poised for victory when the Murrays recovered from a 2-0 deficit in the fourth to lead 6-5 and a chance to serve for the match.
But a Hewitt winner ensured a tie-break and, after a Groth volley had saved a match point, Australia sent the contest to a deciding set.
Further resiliency from Hewitt and Groth saw them fightback from 3-0 down in the fifth to level matters at 3-3, however, following a series of holds, Great Britain finally made the deciding breakthrough.
Singles world number three Andy Murray blistered a cross-court forehand to set up two match points, with only one required as Groth sent a volley long.
Andy Murray will look to wrap up a place in the final when he faces Bernard Tomic on Sunday, with Dan Evans set to face Thanasi Kokkinakis if required.
Argentina are now the favourites to face Britain or Australia in the final after Carlois Berlocq and Leonardo Mayer defeated Belgium's Ruben Bemelmans and Steve Darcis 6-2 7-6 (7-2) 5-7 7-6 (7-5) to go 2-1 ahead.
After losing the first set, Bemelmans and Darcis showed impressive character to fight back from 5-2 down to take the second to a tie-break, only for Berlocq and Mayer to win the shoot-out in comfortable fashion.
Both pairs spurned break-point opportunities in the third before Bemelmans and Darcis broke the Argentine duo to to love for a 6-5 advantage.
They then saved two break-back points to take the third and looked to be heading towards a decider after capitalising on another poor service game from the South Americans.
But Mayer and Berlocq broke back immediately after going 4-3 down and edged a tightly contested fourth set on a tie-break by converting their second match point.