The 21-year-old Chung, who stunned six-time winner Novak Djokovic in the fourth round, was forced to call it quits two games after being treated for a blister on his left foot.
The Swiss maestro now has a 30-13 win-loss record in Grand Slam semi-finals and has yet to drop a set in this year’s Australian Open.
I thought the first set was normal,” Federer said. “I couldn’t tell what was going on with my opponent.
“In the second set, I felt he was getting slower. He’s had a problem with the blister. It hurts – a lot. At some point it’s too much and you make things worse.
“Clearly I’m happy to be in the final – but not like this. He’s had such a wonderful tournament.” I know how hard it is to win in any major,” he said after his match against Chung. Years ago I ran into the Novaks and Rafas along the way and they blocked me for some time.
Federer, chasing his 20th grand slam title, will face Croatian sixth seed Marin Cilic in Sunday’s final.
Federer leads 2014 US Open champion Cilic 8-1 in their meetings.
Federer’s road to the 2018 Australian Open final
1st round: beat Aljaz Bedene (SLO) 6-3, 6-4, 6-3
2nd round: beat Jan-Lennard Struff (GER) 6-4, 6-4, 7-6 (7/4)
3rd round: beat Richard Gasquet (FRA) 6-2, 7-5, 6-4
4th round: beat Marton Fucsovics (HUN) 6-4, 7-6 (7/3), 6-2
Q-F: beat Tomas Berdych (CZE) 7-6 (7/1), 6-3, 6-4
S-F: beat Chung Hyeon (KOR) 6-1, 5-2 retired
Hungarian Timea Babos and France’s Kristina Mladenovic claimed their first grand slam title together, defeating Russian pair Ekaterina Makarova and Elena Vesnina 6-4 6-3 in the women’s doubles final.
India’s Rohan Bopanna and Timea Babos of Hungary progressed to the mixed doubles final, defeating Brazilian Marcelo Demoliner and Spain’s Maria Jose Martinez Sanchez 7-5 5-7 (10-6) on Margaret Court Arena.
The fifth-seeded pair will meet Croatian Mate Pavic and Canada’s Gabriela Dabrowski in the final.