Federer, who will be 35 next year, won his last title of five in New York back in 2008.
But at the presentation ceremony, he had no intention of following women's champion Flavia Pennetta by announcing his retirement.
"I'm very pleased with where my game is at," said Federer, who won the most recent of his 17 majors at Wimbledon in 2012.
"It feels like it has moved in the right direction. I'm back in the final for the first time since 2009. I love the sport. I've got a lot of passion.
Federer was bidding to become the oldest champion in New York since 35-year-old Ken Rosewall in 1970.
But he failed to breach Djokovic's formidable defensive skills, managing to convert just four of 23 break point opportunities.
The Swiss had come into the final not having dropped a set at the tournament and having lost serve just twice in 82 games.
He had also strung together 28 consecutive sets in a run stretching back to the Wimbledon final in July, which he also lost to Djokovic.
Federer, the world number two, still believes he can still punch his weight at the highest level, with his five tour titles so far in 2015 only serving to boost his confidence.
"But Novak did a great job of fending them off. It was a tough night."
"I am playing a good year. I'm playing good tennis. I am happy with where my level is at. I'm able to beat the best players regularly."
Federer said his success at the Cincinnati Masters on the eve of the US Open convinced him that he still had enough left in the tank.
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