With most of the focus on Gauff, Sabalenka had been quietly and efficiently getting on with business.
The 25-year-old, who will become the new world number one after the tournament, had not dropped a set on her way to the last four but had to find a different way to win against Keys.
Keys, who was the runner-up in 2017, showed her pin-point quality from the baseline and hit an array of winners in a one-sided opening set.
By contrast, Sabalenka was spraying the ball and produced a series of unforced errors to benefit her opponent.
Sabalenka had won only one of her previous six Grand Slam semi-finals, including defeats when she held leads at this year’s French Open and Wimbledon.
Flinging a racquet towards her team early in the second set illustrated her frustration and her body language throughout indicated her stress levels.
But she showed extraordinary determination to fight back and earn what must be one of the most satisfying wins of her career.
“Somehow, I don’t know how, I turned around this match and it really means a lot to be in the US Open final for the first time,” she said.
