It is Keys's first major title, having reached a second Grand Slam final eight years after her first in New York. She lost on that occasion to Sloane Stephens
The American battled injury and self-doubts last year and curtailed her season early, in October, to get married.
The 19th seed powered through a thrilling semi-final on a final-set tiebreak against the world number two 5-7, 6-1, 7-6 (10/8) after Sabalenka had earlier swept aside Paula Badosa in straight sets.
Aryna Sabalenka fought back from the brink of defeat to overcome American 17th seed Madison Keys in a third-set tie-break and line up a US Open final against home favourite Coco Gauff.
Sloane Stephens stun two-times U.S. Open winner Venus Williams to ensure a new grand slam champion is crowned at the Flushing Meadows while Madison Keys hammer CoCo Vandeweghe to set up an all-American final.
Caroline Wozniacki, derailed this year by an ankle injury which sent her career into a tailspin, and Anastasija Sevastova, who quit the sport three years ago, set up an unlikely US Open quarter-final face-off.
