It was during the second set of the men's semi-final match when the Serbian, who was down 3-1 when the commotion began, alerted officials to the stricken fan slumped over in his seat. No. 3 seed Federer, who won the first tiebreaker set 7-6 looked on.
Even for the boisterous tennis crowds in Flushing Meadows, the excessive whistling seemed loud.
After the eagle-eyed Djokovic knew that the fan was being attended to, he immediately turned his attention back to his game.
Djokovic was asked by the referee if he wanted to continue play. Djokovic grabbed a ball and pressed on. He went on to win the next serve.
The diversion seemed to motivate Djokovic, who up to that point seemed sluggish and outplayed. He rallied slightly against his Swiss opponent, only to fall short in the second set, 6-4.
Djokovic's momentum carried over to a 6-3 third set win and now it was Federer who started to look tired.
Djokovic forced a decisive set five by winning 6-2, then continued to hit balls fast and long while Federer couldn't get a point on any of his serves.
In the fifth set, the expressionless Federer got the crowd on his side, looked fresher and started to spray balls beyond Djokovic's reach.
The momentum continued to volley back-and-forth between the two players.
In the end, it was the Serbian's turn to get the crowd on his side. The cheers pushed Djokovic to an improbable comeback after he fell behind in, what seemed like, an insurmountable 5-3 hole.
The classic match was finally won in a tiebreaker by Djokovic, 7-5 in the fifth.
After the epic battle, Djokovic lauded the "noise" at Arthur Ashe Stadium and mustered up enough energy to dance with the 23,500 jubilant fans.
The nearly four-hour match's start was delayed by rain for about an hour. Djokovic will meet either No. 2 Rafael Nadal or No. 4 Andy Murray in the finals on Monday.