Swiatek was in imperious form in Melbourne, losing just 14 in the five matches before the semi-final.
For context, only three players in history have lost fewer matches going into the last four of the Australian Open, which has seen 128 players draw in the Open era.
However, the four-time French Open winner was defeated by an inspired Keys, determined to win the Australian Open semi-final at his third attempt.
A chaotic start that included five breaks of serve in the first six games set the tone for an awkward night under the Rod Laver Arena roof for Swiatek, who broke for a fourth time to win the first set before capitulating in the second.
Keys needed just 27 minutes to defeat Świątek, winning five games in a row and taking the decisive set.
Both players saved multiple break points as the tension mounted during a gripping, high-quality deciding set.
Swiatek finally converted her eighth break opportunity to take a 6-5 lead, but after failing to capture her first match point, she lost her serve on a double fault.
Keys and the entire match deserved a winner-take-all tie-break, but with the score at 7-5, Świątek seemed to have everything under control.
But in a final impressive display of physical and mental resilience, Keys scored five of the next six points to seal a sensational victory after two hours and 35 minutes.
