He played two more games but, serving at 5-2 behind, Chung abruptly headed to the net to shake hands, taking the packed crowd in Rod Laver Arena completely by surprise.

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 will play Marin Cilic on Sunday in a repeat of last summer’s Wimbledon final - when coincidentally the Croatian was in tears because of blisters - as he bids to become the first man to reach 20 grand slam singles titles.

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 Novak Djokovic in the fourth round, that he possesses the game to trouble the best.

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 the medical time-out, although he was completely unable to match Federer, who bullied the Korean with his forehand.

The near 15-year age gap was the fourth largest in a grand slam semi-final in the Open era, with the younger man having won on the previous three occasions.

But Federer has been tearing up tennis records virtually his whole career and his seventh appearance in the final here sets yet another new mark.