Most analysts had written off Djokovic because of a lack of mental toughness against Federer in particular, besides his struggles this year.
Filled with quality shot making and emotional intensity, this was probably the most thrilling semifinal ever played at the Arthur Ashe stadium.
Djokovic will play the Championship match with well-rested Rafael Nadal, who earlier dismissed Mikhail Youzhny in straight sets (6-2, 6-3, 6-4).
In a most grueling five-set slug fest, the Serb came back from two match points down in the last set (4-5, 15-40) to deny the most anticipated final of the decade between Federer and Rafael Nadal, who have played finals in all the other three Grand Slams except the US Open.
How did Djokovic face those match points and overcome them?
Federer's defense forces Djokovic to shank the second overhead into the net. The Serb overcooks two backhand shots in a row.
He still cannot get his first serve in but on his second serve close to the T-line allows him to run Federer corner to corner and end the point finally hitting a big winner. After a short rally, he rips another forehand winner deep into the corner.
Djokovic celebrates the erasure of the match points with a fist pump. He still has to win two more points to level the set at 5-5.
He drop volleys and follows with a forehand approach shot to earn a game point. Federer deuces a couple of times.
Djokovic serves big and plays 1-2 punch and in the next point forces Federer to slice into the net.
As soon as he holds his serve, Djokovic wins the first point, with a forehand winner after a sterling rally. After 30-30, Federer's wild forehand provides Djokovic to break and he does it in the first instance. Now, he gets his chance to serve out the match.
Djokovic quickly goes down to 0-30 but Federer does him a favor by missing his forehand shot. Djokovic earns another point with a volley at the net after a good rally (30-30).
He misses a backhand wide by a centimeter. Federer cannot capitalize on the break point. He throws forehand long and shanks another into the net.
The Serb finally proved he is mentally tough even at the Slam level, if he had not done so in Basel, where he had beaten Swiss in his home turf last year.
Federer recognized that it was wrong to overlook a player of Djokovic's caliber:
"Honestly, I think he played already well against me the last three times we played here in New York. So it was not like the guy can't play under pressure. He's proven his point, and time and time again. I knew he was gonna be a really tough opponent. The guys who overlooked him don't know anything about tennis, unfortunately."
This entire last game of the match was over-cautiously played (slow but long rally) until Federer decides to go for more to throw his forehand into the net. Djokovic basked in victory.
Djokovic had Federer on the run from the beginning of the first set, earning double break point in the Swiss' first service game and converting one of the triple break points on the sixth game to lead 4-2.
Had Djokovic not blinked to commit a double fault in the seventh game to give Federer a break point, he probably would not have lost the first set.
Djokovic looked nervous once again in the 11th game and missed a number of easy returns, either throwing his shots wide or long from both wings, gifting Federer a break in love and 6-5 lead.
Djokovic mounted a fierce opposition but Federer's big serve earned him a couple of free points and the set.
In the second set Federer lost (1-6), his first serve percentage was bleakly down to 40 and his shots were misfiring more often than Djokovic's (10-4 unforced errors).
Credit to Djokovic who was reading Federer's serve well and winning 30 percent more points on returns.
Like in the second set, Djokovic broke Federer twice in the fourth set (third and fifth games). The Serb simply was at the top of his game, painting the lines again and again, deep into both corners.
Despite serving a lot better (68% first serve in), Federer could not do much to stop the Serb's insurgence.
In the last set, Djokovic did everything better than Federer: He served better, returned better, hit more winners, and committed less unforced errors. More importantly, he held his nerve.
This is the first time the five-time US Open champion has failed to reach the final in six years. Arguably the best hard-court player of the Open Era, Federer was not even expected to lose two sets, let alone the match to the opponent he had beaten three times in a row at the same venue, losing only one set in total.
Even if the aging lion seems to have now handed his safest fortress to the younger generation, Federer is likely to remain a US Open title contender for a couple of years more.
Much to the satisfaction of Djokovic fans, the Serb has finally overcome the ghost of Federer and shown signs that he will be the force he used to be when he won his first Grand Slam back in 2008.
As for tomorrow's final, Djokovic matches very well with Nadal and has 7-3 head-to-head lead on hard court, but they have never played at the Grand Slam final before.
If the Serb is not content with today's victory over Federer and if he can regain full mileage by tomorrow afternoon, he can win his second Grand Slam.
Djokovic has two weapons going into the match tomorrow: First, he is a better hard court player than Nadal; second, he has the confidence to beat the Spaniard.
Nadal also has two things going: He is well rested, and he believes he cannot be defeated at the Grand Slam final.
Djokovic responded jokingly, "I have to, you know [have enough gas in the tank]. It's finals of the Grand Slam. I've been in this situation before in 2007, so I'm more experienced than Rafa."
The Serb is aware of the situation: "After such a big win it's hard [to stay calm], because you're so happy. But, you know, you're playing tomorrow finals of a major against the best player in the world. You want to give your best, and anything I have left I will leave to the court tomorrow. I hope luck will be on my side."
A full-throttle battle between Career Slam and second Grand Slam is highly anticipated, especially since the weather-postponed final will now be played on Monday.
Last word goes to the winner: "I mean, it would be such a big success and achievement beating Roger in semis and then Nadal in finals. I cannot even describe it."