Madison Keys saved a match point before stunning world number two Iga Swiatek to set up an Australian Open final against two-time defending champion Aryna Sabalenka.
Keys claimed a shock 5-7 6-1 7-6 (10-8) victory over five-time Grand Slam champion Swiatek to reach the second major final of her career.
Poland's Swiatek served for the match at 6-5 in the third set and held match point but Keys saved it, broke back and then recovered from 7-5 down in a fraught tie-breaker to advance.
That came after Belarusian top seed Sabalenka earned the chance to become the first woman in 26 years to win three successive Australian Open titles with a commanding 6-4 6-2 win over close friend Paula Badosa.
While Keys has achieved a career-best Australian Open run, Sabalenka is aiming to emulate Martina Hingis' three straight titles between 1997-99.
"I have goosebumps. I am so proud of myself and my team that we were able to put ourselves in such a situation," Sabalenka said.
"If I put my name in history it will mean the world for me. I couldn't even dream of it."
Swiatek's loss also means Sabalenka will retain the world number one ranking after the tournament.
Keys stuns Swiatek to make first Melbourne final
Swiatek had been in imperious form in Melbourne, losing just 14 games across her five matches before the semi-finals.
For context, only three players in history have dropped fewer games in making the last four at the Australian Open in a 128-player draw during the Open era.
However, the four-time French Open winner was undone by an inspired Keys, determined to win an Australian Open semi-final at the third attempt.
A messy start featuring five breaks of serve in the opening six games set the tone for an awkward night under Rod Laver Arena roof for Swiatek, who broke for a fourth time to take the first set before capitulating in the second.
Keys took just 27 minutes to overwhelm Swiatek reeling off five straight games to force a deciding set.
Both players saved numerous break points as the tension ramped up during an enthralling, high-quality deciding set.
Swiatek eventually took her eighth opportunity to break for a 6-5 lead but, after failing to take her first match point, conceded serve with a double fault.
A winner-takes-all tie-break was what Keys and the match deserved but Swiatek looked to be in control of it at 7-5 up.
However, in a final impressive show of physical and mental resilience, Keys won five of the next six points to complete a sensational win after two hours and 35 minutes.
History-chasing Sabalenka puts aside friendship
Aryna Sabalenka beat Qinwen Zheng in straight sets in last year's Australian Open final
Top seed Sabalenka previously described Badosa as her "soulmate" but she successfully put aside their off-court relationship to continue her pursuit of a third successive triumph at Melbourne Park.
Sabalenka, winner of both the Australian Open and US Open last year, quickly went a break down against Badosa, and the Spaniard was within a point of taking a 3-0 lead.
But Sabalenka emphatically hit back, winning 12 of the next 16 games to record her 20th straight victory at the tournament.
The two barely exchanged a glance as they focused on their respective tasks but they shared relieved smiles after Badosa fell early in the second set before raising a reassuring thumb as she lay on the court.
What started as a fiercely-contested match began to drift away from first-time major semi-finalist Badosa when she gifted Sabalenka control of set two, conceding a break of serve with back-to-back double faults
Another in the fifth game then allowed Sabalenka to break for a second time with one of her 32 winners, before she served out victory in one hour and 26 minutes.
Sabalenka, later seen consoling Badosa in the players' area, said: "It was a super tough match against a friend, I'm super happy to see her at her highest level."
BBC