The world No2 took three hours to seal a 6-2 7-6(3) 6-4 win against his Argentinian opponent, but the Serbian insisted the match was a much closer affair than the scoreline suggested.
“It was a straight sets win but it was much closer and tougher than the score indicated,” said Djokovic.
“Whoever saw the match would understand we played almost three hours. The second set was crucial to get a two-set advantage, and to feel better coming into the third set.
“It could have gone either way. It was very long and had a lot of important and exciting rallies. The crowd got into it and it was a lot of fun in one way to be part of this exciting match.”
The 25-year-old is expecting a very difficult match against Ferrer, who won in five sets against Janko Tipsarević, for a place in the final at Flushing Meadows.
“David is a fighter. He is one of the biggest competitors we have in the game,” added Djokovic.
“People overlook him, but he has been one of the most consistent players on the tour. He plays great on every surface and you need to earn your points against him.”
Djokovic also believes that he has learned how to handle the tag of defending champion since winning his first Grand Slam title in Australia in 2008.
“The first time I defended my Grand Slam title was in Australia in 2009,” he added.
“It was a difficult period for me because I faced the challenge for the first time, pressure got the better of me, but I’ve learned over the years how to cope with the pressure and understand the game better.
“Right now I try to focus on every game individually, but it definitely has a special feeling about it, coming as a defending champion.”