Higgins was left stunned after world number 21 King won four of the last five frames to take the match 6-5 and book his place in the quarter-finals.
"There are just some things you can't explain where you malfunction on every single shot and cannot pot a ball," Higgins said after the loss.
"When you collapse like that, there's only one way you're going to go in this game."
King had not gone beyond the second round of the UK Championship since 2005 before his run this year and is delighted to still be involved.
"I had a couple of bad shots at the beginning of the frame and broke off like I had never played before and John had a couple of chances," he said.
"But with my first chance, I stayed positive and was in a pretty good position. I was pretty calm and confident and made sure I didn't miss any easy balls. It's nice to still be involved."