The South Korean was one set down and close to losing the second when he strolled over to the umpire to withdraw. 

Chung had a medical time out midway through the second set as medical staff tended to his painful feet. 

However, it was too much for the 21-year-old who later called it a day meaning Federer will go for his 20th Grand Slam on Sunday. 

WERE BLISTERS THE REAL REASON BEHIND WITHDRAWAL? INTERESTING THEORY SUGGESTED

“I thought the first set was normal. I couldn't tell what was going on with my opponent,” Federer said on court after the match.  

“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.”

Take a look through the gallery above to see the gruesome pictures of Chung Hyeon blisters.

Pundit Annabel Croft says she noticed that Hyeon Chung was having problems with his movement before he got treatment for blisters.

And the tennis pundit admits she initially thought it was a whole different kind of issue relating to the 21-year-old not having enough self-belief against Roger Federer.

Chung had six minutes of treatment courtside for the blisters on his left foot before he quit in the next game after less than an hour on court.

She said: "I actually thought while watching it that his movement was starting to get a bit restricted and shackled.

"I wasn't sure if it was him not believing in himself but I think those blisters were far worse than any of us could have imagined.

"It's such a great shame. Everyone has paid for their tickets out there, the TV audiences, everybody really because I think we all bigged it up but it was a shame we didn't get a really competitive semi-final."