Maybe that's why the normally jovial Serb was a little grumpier than usual during the fortnight in New York.
But Djokovic had the last word at the year's last major Sunday, defeating Federer 6-4 5-7 6-4 6-4 -- disappointing the majority of fans assembled at Arthur Ashe stadium -- to cap one of the best seasons in grand slam history.
"It's been an incredible season," said Djokovic. "I'm very fortunate to experience a great success this year."
The finale pushed back by three hours due to rain, the contest began at about 7:15 p.m. local time, making for an impromptu night session.
And New York sure loves its night sessions.
On this occasion, perhaps the combatants didn't.
What unfolded was tense, turbulent tennis, no doubt thanks in part to cooler, slower conditions that particularly didn't do Federer any favors. It was by no means chilly, but the sweltering heat of the first week and a half dissipated.
"Me, I would have preferred if it was hotter," Federer's co-coach, Severin Luthi, told reporters. "The ball, it was slower and all that but we can't choose. It's like that."
Djokovic hardly looked comfortable most of the affair, while Federer will rue going 4-for-23 on break points. His forehand crumbled on pivotal points and the ultra-aggressive tactics he employed -- with success -- in the past months fizzled against the world's top-ranked player. The final three games of the third proved decisive.
"I had too many break point chances," Federer, who committed 54 unforced errors, said. "Some of them I could have done better, should have done better.
"Maybe I haven't played this offensive for a very long time and that's maybe the reason as well why maybe I was slightly shaky when it came to the crunch time on the break points."
In the first three sets, Federer's forays to the net failed to lead to much success.
For all the discussion of Williams, and it was justifiable, Djokovic ends 2015 with a better record at majors than the American, 27-1 compared to 26-1. If it wasn't for Stan Wawrinka at the French Open, the 28-year-old would be departing the Big Apple as the first man to achieve the Grand Slam since Australia's Rod Laver in 1969.
He'll take three in a season, matching his 2011 haul, especially since it gave Djokovic a landmark 10th major. Forecasting the final tally of Grand Slams for the "Big Four" has become de rigueur in recent years, with the bulk of the attention restricted to whether Rafael Nadal can tie Federer's men's record of 17.
But now, with Nadal slumping and Djokovic soaring, the latter needs four majors to draw level with the Spaniard.
"I will continue on with the same kind of lifestyle, same kind of approach," said Djokovic. "I think that kind of approach brought me to where I am today. Hopefully this kind of approach will give me longevity and that I can have many more years to come."
Federer hadn't surrendered a set since losing in the Wimbledon final in four sets to Djokovic, introducing his controversial SABR -- sneak attack by Roger -- tactic and serving with aplomb. Some were saying Federer, ranked second, was playing the best tennis of his career.
Once again, though, it was Djokovic who stopped him.
Although Federer defeated Djokovic last month on Cincinnati's slicker hard courts, he has now lost seven of their last nine grand slam duels. His drought at majors extends to three years.
A retractable roof will be in place in time for next year's tournament, so perhaps the precipitation Sunday afternoon was Mother Nature's parting gift. Rain, mind you, also affected finals from 2008-2012.
Federer signaled his intent with an ace down the middle on the first point, initiating a roar from the spectators who were chuffed to finally see tennis.
Djokovic struggled with his footing throughout and took a nasty spill as he attempted to consolidate a break in the first. Running down a Federer volley at 0-15, he slipped changing directions. Djokovic was fortunate not to roll an ankle, yet the damage was done with a scraped elbow, back of his hand and leg.
Rattled, Djokovic surrendered his serve for 2-2 and Federer duly held to love.
Visited by the trainer during a changeover after the fifth game, Djokovic seemed to settle and re-established his advantage, twice breaking the Swiss.
The SABR, when Federer authors a half-volley, or close to it, backhand return on his opponent's second serve, made its debut in the second game of the second set, when he blew five of the break points. Becker said during his day, in the 1980s, if a player tried that, they'd have been plunked by the server.
No one has come close to doing that to Federer, but Djokovic's reply to the ploy later in the second -- a sumptuous backhand lob -- was perfect.
Federer's serving difficulties eased in the second, a contrast to Djokovic.
In an absorbing 10th game lasting more than 13 minutes, Federer squandered a set point with Djokovic well stranded. The latter was mightily relieved.
Such a gaffe against Djokovic would usually be insurmountable, but Djokovic didn't take advantage of his second chance.
When Federer misfired on another forehand on set point at 6-5, the backhand -- for so long considered his less potent side -- came to the rescue: A crosscourt drive befuddled Djokovic.
Federer leaped in celebration and the New York crowd lapped it up, reminiscent of the atmosphere four years ago at Roland Garros when the two men collided and Federer snapped Djokovic's 41-match winning streak to begin 2011.
The crowd was "unbelievable," Federer said. "Were they better than ever? Possibly. Was it louder than ever? Maybe. It was unreal."
He enjoyed the momentum and despite conceding serve at 1-1 -- the forehand, once again, came unstuck -- Djokovic donated two double faults, one on game point, and it was 2-2.
Neither player was producing vintage stuff, the miscues mounting. Holding break points after Djokovic led 40-0 on serve in the eighth game, the forehand faux pas from Federer ensued. It was 4-4.
Seizing the initiative by breaking for 5-4, Djokovic subsequently trailed 15-40 on serve but hung on, a service winner and backhand error taking him to the respite of deuce.
"When I managed to break at four all and managed to hold after saving (a) couple break points at 5-4 and winning the third set, that obviously gave me a huge wind in the back, and I managed to play really well after that," Djokovic said.
This time there was no rallying from Federer. Deflated, he was broken to begin the fourth and soon Djokovic increased his lead to 5-2.
"I should never have been down in the first place two sets to one and 5-2," said Federer. "That was a bummer there."
To seal Federer's break-point misery, Djokovic fended off three break points in the final game.
A return rifled long from Federer, Djokovic grabbed his second U.S. Open title after falling in four of his previous five finals in the Big Apple.
"I have not been so successful in the finals here as I am maybe in Melbourne, for example, or Wimbledon," said Djokovic. "But I'm glad that this tradition of losing in finals is broke now."