Gauff, 19, produced a gutsy performance on the Arthur Ashe Stadium court to win 2-6, 6-3, 6-2 in 2hr 6min to complete a fairytale transformation in her season's fortunes.

The sixth seed from Florida had gone into the final as the underdog against the hard-hitting second seed Sabalenka, who will become world number one in next week's rankings.

But with both players making a slew of mistakes throughout an error-strewn final watched by a record crowd of 28,143 it was Gauff who held her nerve when it mattered to seal a deserved victory.

The win completed a remarkable turnaround for Gauff, who was left distraught after a first round exit at Wimbledon in July.

However she bounced back to win titles in Washington and Cincinnati and has now landed the biggest win of her career, after a shattering loss at the French Open last year.

"It means so much to me," an elated Gauff said afterwards. "I feel like I'm a little bit in shock in this moment.

"That French Open loss (last year) was a heartbreak for me. That makes this moment even sweeter than I could imagine."

Gauff, the third American teenager to win the US Open after Tracy Austin and Serena Williams, also used her victory speech to thank those who doubted her talent.

"Honestly thank you to the people who didn't believe in me," she joked. 

"To those who thought they were putting water on my fire, they were putting gas on my fire and I'm burning so bright right now."

- First set struggle -                  

Gauff was in trouble in the opening game, Sabalenka breaking her straight away with a rasping backhand that drew a roar of "Come On!" from the Belarusian.

She held easily to take a 2-0 lead but Gauff then took advantage of a shaky service game from Sabalenka to break at 2-2 in the fourth.

The Belarusian double-faulted twice to allow Gauff to get back on level terms.

But that hard-won parity was surrendered in the next game as Sabalenka broke back to go 3-2 ahead.

Sabalenka then wobbled on her own serve once more as Gauff eked out two break points in the sixth game.

But Sabalenka got it back to deuce with an ace and then took a 4-2 lead with an emphatic smash.

Gauff's problems on serve continued and Sabalenka broke for the third time to race 5-2 ahead, and she duly wrapped up the set by holding in the next game.

Yet with the match threatening to become a rout, Gauff finally clicked into gear in the second set, making fewer unforced errors and ironing out the kinks in her serve.

Instead it was Sabalenka who began to show signs of brittleness as the tension mounted. She double-faulted to hand Gauff the only break of the set and a 3-1 lead.

Gauff fended off a break point in the next game to hold for 4-1 and went on to hold for the remainder of the set to level the match when Sabalenka smacked a forehand long.

The momentum remained firmly with Gauff in the final set and she secured another crucial break in the opening game when she put away an underhit Sabalenka lob with a smash.

Gauff then held easily for a 2-0 lead as Sabalenka struggled to regain any semblance of composure.

She coughed up four unforced errors to gift Gauff a break and a 3-0 lead, and the American then held with ease to go 4-0 up.

Sabalenka stopped the rot by holding serve in the fifth game, before taking a medical timeout to receive treatment on her left thigh.

Gauff was in no mood to let her grip on the match slip though. 

Although Sabalenka held and broke Gauff to cut the lead to 4-2, Gauff hit back when Sabalenka double-faulted to present a break point.

Gauff cashed in to break and grab a 5-2 lead and then swept to victory in the next game, holding to love with a backhand winner.