Cilic comfortably defeated Yorkshireman Kyle Edmund on Thursday while Federer did not even have to complete his semi-final after Chung Hyeon retired with severe blisters trailing 6-1 5-2, to the shock and disappointment of the crowd on Rod Laver Arena.
There was a smattering of boos, but Federer had huge sympathy for his opponent.
He said: “I thought the first set was kind of normal, I couldn’t tell what was going on with my opponent.
“In the second set I started to feel he was getting a bit slower, fighting with the blisters.
“I’ve played with blisters in the past and it hurts a lot. At one point it’s too much. It’s better to stop.
“This feels bittersweet. I’m incredibly happy to be in the finals, but not like this.”
Federer continues to reset the record books and will go for a 20th grand slam title in his 30th final.
Tomorrow’s match will be a repeat of last summer’s Wimbledon final – when coincidentally Cilic was in tears because of blisters.
“But they have faced each other more recently and in more unusual surroundings.
Federer said of their holiday practice date: “I was there first and he arrived later on the island. I was told that Marin was coming. I was like, ‘Oh, that’s cool’.
“When he arrived, I didn’t want to bother him. He didn’t want to bother me.
“After two days, he wrote to me, ‘I’m here, too, in case you want to catch up and stuff, let me know’. I was like, ‘Sure, let me know if you want to hit’.
“He was eager to hit because it’s good to stay in the rhythm for both of us.
“We also met up later for drinks, I met his fiancee.
“We had cake together, my whole family and him. We had a good time.
“We actually went to practice twice for 45 minutes. It was great fun.
“No coaches, no nothing, just the two of us on the court hitting balls. It was just nice and laid back.
“To get to know the man behind the tennis player, I guess.”
While Federer is bidding for his record-extending 20th grand slam title, Cilic is seeking his second, his first having come three and a half years ago at Flushing Meadows.
“It’s great to again be in the final, giving myself another opportunity to win a grand slam,” said the 26-year-old 2014 US Open final winner.
“I’m playing very good tennis, and definitely very excited for the rest of 2018.”
On facing Federer, he added: “Roger as a player has been one of the most successful at these stages of grand slam, and also at grand slam level I think his performances were such a long period of time, especially when it gets to quarter-finals, semi-finals, finals, he’s getting better and better and better and playing great tennis.”
In yesterday’s semi-final, Chung was bidding to become just the third Asian player to reach a grand slam final after former Australian Open champion Li Na and Kei Nishikori, and he had shown, particularly with his victory over six-time champion Novak Djokovic in the fourth round, that he possesses the game to trouble the best players in the world.
The Korean will have bigger and better days at slams in the future, but this was a difficult experience from the moment he dropped serve in the opening game.
He gave no indication that he was struggling physically until calling for a medical time-out at 1-4 in the second set, although he was completely unable to match Federer, who bullied the Korean with his forehand.
It emerged afterwards that Chung had been struggling with multiple blisters for several days and an injection to try to numb the pain was not successful.
The 21-year-old said: “It really hurt. I can’t walk any more. I think I did the right thing. If I play bad on the court, it’s not good for the fans as well. I’m happy to be able to make semis in a grand slam. I want to be stronger next year.”
Britain’s Gordon Reid and Alfie Hewett lost the Australian Open men’s wheelchair doubles final in straight sets to French second seeds Stephane Houdet and Nicolas Peifer.
The reigning US Open and Wimbledon champions were beaten 6-4 6-2 in one hour and 21 minutes at Melbourne Park yesterday.