Keys served for the match in the second set but incoming world number one Sabalenka overhauled the 2017 runner-up to win 0-6, 7-6 (7/1), 7-6 (10/5) to reach her second Grand Slam final.
“She played incredible tennis, it was just another level. Somehow, I don't know how I turned around this match,” said Sabalenka, the second seed.
“It really means a lot to be in the final of the US Open for the first time.”
Sabalenka rode an emotional rollercoaster, angrily remonstrating with her coaching team at times, before being able to sheepishly laugh off celebrating prematurely in the decisive tie-break.
“I thought that we play tie-break up to till seven,” she said. “I was just all over the place.”
The Australian Open champion will bid for her second Grand Slam title on Saturday against the 19-year-old Gauff. The American holds a 3-2 edge but this will be their first meeting at a major.
“She's an unbelievable player,” said Sabalenka. “The crowd will be supporting her a lot. I will do everything I can. I'll be fighting for every point and I will do my best.”
Sabalenka handed Keys a break in the first game with a double fault and her troubles were compounded when she again dropped serve after leading 40-0 two games later.
Keys shook off two break chances before a wayward Sabalenka faltered on serve for a third time, allowing her opponent to wrap up the set with ease.
Sabalenka sarcastially raised her arms in celebration as she held to finally get on the board to start the second set.
Keys continued to pump winners with her powerful forehand and broke again for a 2-1 lead, Sabalenka drawing a code violation after smacking a courtside barrier and hurling her racquet towards her coach.
The Belarusian briefly got back on serve before Keys struck once more, further infuriating an irate Sabalenka who had stormed into the semi-finals without losing a single set.
Sabalenka broke Keys to love to stay in the tournament, winning 12 points in a row to move 6-5 ahead, but she couldn't convert two set points as the American steadied herself to force a tie-break.
However Sabalenka won that easily to send the match to a deciding set, before which Keys took a medical timeout to receive treatment to her upper left leg.
Keys withstood an early break point and the first five games went on serve before a loose game from Sabalenka handed Keys the initiative at 4-2.
Sabalenka dug deep and overpowered Keys to retrieve the break straight away, surviving two more break points to level at 4-4.
The Belarusian took charge in the tie-break, leading from the front before securing a second win over Keys in as many Grand Slams.