Top seed Ashleigh Barty was knocked out of the 2020 Australian Open by America's Sofia Kenin on Thursday.
The 21-year-old beat the French Open champion 7-6 (6), 7-5 to book a spot in her first Grand Slam final.
Kenin will meet unseeded former world No. 1 Garbine Muguruza, who upset No. 4 seed Simona Halep with a 7-6 (8), 7-5 victory.
The women's final will take place on Saturday on Rod Laver Arena at Melbourne Park.
In the first men's semi-final, defending champion Novak Djokovic survived a blistering start from Roger Federer to eventually prevail in straight sets.
Australian Open - Thursday Results
(14) Sofia Kenin bt. (1) Ashleigh Barty: 7-6 (6), 7-5
Garbine Muguruza bt. (4) Simona Halep: 7-6 (8), 7-5
(2) Novak Djokovic bt. Roger Federer: 7-6 (1), 6-4, 6-3
Despite both women's semi-finals being decided in two sets, they were tight contests.
Barty, attempting to become the first Australian women to reach the final since 1980, had two set points in both sets but failed to convert.
In the opening set she was dominating the tiebreak only for Kenin to wrestle the momentum with two key winners.
The American was all but gifted the opener when Barty netted a return to a second serve.
The home favourite composed herself and broke early in the second to lead 2-1. But she stuttered again when serving for the set at 5-4 ahead, and Kenin powered to victory in one hour, 45 minutes.
Before the 2020 Australian Open, Kenin's best Grand Slam result was a fourth-round berth at the 2019 French Open, and she had failed to get past the second round in two previous visits to Melbourne.
Muguruza, meanwhile, is no stranger to major victories having won the French Open in 2016 and Wimbledon in 2017.
In nine Grand Slam appearances since that Wimbledon triumph, though, she had suffered two first-round exits, three second-round defeats, three fourth-round losses and made just one semi-final appearance.
In the opening Grand Slam of 2020, though, she has looked back to her former best, and the Spaniard's battle against Halep was superb.
Muguruza edged a 67-minute opening set in a monumental tiebreak.
The twists and turns of the opener were almost matched in the second set, when Halep and Muguruza exchanged five breaks of serve.
Crucially, it was the 26-year-old who struck when it mattered most to avoid another tiebreak and book herself a spot in a maiden Australian Open final.
Djokovic's clash with Federer was a topsy-turvy affair for the first hour.
The Swiss icon had to save two break points and serve three aces simply to hold serve in the opening game before breaking Djokovic for 2-0 with a vintage backhand pass.
The defending champion broke immediately back before dropping serve again, and Federer's dominance was such that he looked to be running away with the first set.
However, serving for the set at 5-3, the 38-year-old buckled.
A double fault let Djokovic in to get the set back on serve, and the Serb duly dominated the subsequent tiebreak.
After 63 minutes of almost complete Federer dominance, he found himself a set down, and with the fast start not having paid off, he was on the back foot physically and mentally.
The second and third sets were almost entirely controlled by Djokovic, who took the crucial breaks seemingly when he wanted them and completed victory in two hours, 18 minutes.