Federer and world No 1 Novak Djokovic had been expected to set up a repeat of the Wimbledon final but instead both were the victims of huge upsets.
After an off-colour Djokovic’s four-set loss to Nishikori, Cilic produced a remarkable performance in a one-sided 6-3 6-4 6-4 victory over the 17-time grand slam champion.
It is the first time since the Australian Open in 2005 that a slam final will not feature one of Federer, Djokovic, Rafael Nadal or Andy Murray and the first time since the French Open in 2002 that both players are ranked outside the top 10.
Cilic said: “It’s an amazing day for me. I feel amazing. To be able to play like this, I never dreamed of it. I think today was my best performance ever in my career.”
Of the final, he added: “I think it’s going to be a sensational day for both of us. I’m extremely happy to be in the final for the first time. I’m just going to enjoy it, be happy and try to win.”
Pundits have been pronouncing the end of the ‘big four’ era for the last couple of years but it is only here that results have really backed that up for the first time.
Federer made no excuses, saying: “It’s fairly simple: I think Marin played great. I maybe didn’t catch my best day, but I think that was pretty much it in a nutshell.
“I was feeling good. I just think if I could have stayed longer with him in the first set, I felt like there was a proper match going on. But him playing with the lead, he played with no fear and just full-out confidence. He served great when he had to. Credit to him for just playing incredible tennis.”
With Djokovic already out of the tournament, Federer would certainly have seen this as a great chance to extend his record with an 18th title.
The 33-year-old has won more matches than any other man this season but it is more than two years since his last slam win.
“I’m just really disappointed after how well I have played this season, especially here also at the tournament,” said Federer. “I really felt like I could win this tournament.
“I don’t need (another title) to be more happy or anything. But the moment itself, it would mean a lot.
“I’ll give it a go again in Australia – I hope to be healthy there. I enjoy playing there. It’s been one of my most consistent slams. I hope to get another chance at it. I can’t do more than try really hard, which I’m doing.”
Djokovic was bidding to make his fifth consecutive US Open final but Nishikori played superbly to win 6-4 1-6 7-6 (7/4) 6-3.
The 24-year-old world No 11, who also becomes the first Asian man to reach a slam singles final, said: “I don’t know what’s going on.
“I was a little bit tight, especially with my first semi-final in a grand slam. It’s just an amazing feeling, beating the No1 player and playing much better than the last couple of matches.
“I’m so happy. It was tough conditions but I guess I love to play long matches and I hope I can recover well for the final.”
A downbeat Djokovic gave a frank assessment of his display afterwards, saying: “Other than that second set, my game today was not even close to what I wanted it to be. A lot of unforced errors, a lot of short balls. I just wasn’t myself.
“I felt like I could have gone at least a step further here, but it was disappointing today. It’s sport. I have to congratulate the better opponent and move on.”