Ki played 90 minutes in his country's 3-0 friendly loss to Japan in Sapporo last Wednesday before putting in another full shift last Saturday for Celtic in their 5-1 win over Dundee United, scoring a goal in the process.
Styled as the 'Koren Gerrard', his energetic and hardworking playing style only exacerbated his fatigue.
The 22-year-old, speaking to STV, said: "I was seeing two footballs at times. I can't even remember what my goal looked like because I was too tired,
"There was such a big time difference that when I played the game it was actually time for me to sleep.
"I didn't expect to get a full game but it is a privilege to play, especially at Celtic Park, so I had to keep going, I was never going to ask to come off."
He won't be getting much time to rest as Celtic take on Swiss club Sion Sitten on Thursday in the first leg of their Europa League qualifying round.
On the bright side, he won't have to travel again as the Scottish side play at their home ground, Celtic Park.
He added: "The couple of days rest at the start of the week will do me good because then I will be ready again for Thursday. We all want to play in Europe and we have to show we are capable of doing that.
"It will be great experience for all of us if we qualify. There is a big difference because we have been together for a year now and we all know each other better."