The 23-year-old came through a tense battle between the world's top two players 6-3, 7-6 (7/4), 6-3 on Rod Laver Arena, raising his arms in the air and looking to the sky in celebration.
The victory earned the Italian world number one his third Grand Slam crown.
The former world number one responded on Saturday, posting a photo of an MRI taken of his left hamstring on the social media platform X with the caption: "Thought I'd leave this here for all the sports injury 'experts' out there."
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
Sinner's coach Darren Cahill said it was no surprise for him to see his young charge back in the final, but they were wary of the threat posed by Zverev's overarching desire to finally win a Slam.
The American battled injury and self-doubts last year and curtailed her season early, in October, to get married.
Swiatek said she made far too many mistakes against Keys on Rod Laver Arena and hoped to draw lessons from it.
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.
The Italian top seed faces Ben Shelton on Friday, just two matches away from a first successful Grand Slam title defence after winning his maiden major crown in Melbourne last year.
Novak Djokovic has denied "absurd" allegations in an Italian newspaper that he "wanted to lose" a match at the Paris Masters in 2007.
More players reveal match-fixing approaches on Tuesday as the Australian Open Grand Slam tournament came under close scrutiny following claims that corruption in tennis was widespread.
Novak Djokovic and Roger Federer are poised to become the first $100 million men in tennis in 2016, in a gripping sub-plot to the new Grand Slam season.
"Fantastic Friday" at the US Open sees Serena Williams, Novak Djokovic and Roger Federer in starring roles.
Rafael Nadal remains on course for a 10th French Open title with a straight-sets victory over fellow Spaniard Nicolas Almagro in the second round.
Andy Murray and Heather Watson face familiar opponents in the French Open second round on Thursday.