Cameron Norrie pulled out of the Korea Open less than an hour before he was due on court to play his quarter-final against American Jenson Brooksby.
The ATP confirmed the second seed was unable to play because of illness.
Norrie, 27, said he was suffering from jet lag after beating Japan's Kaichi Uchida 6-2 6-2 in the previous round.
The British number one has risen to eighth in the world and is chasing a place at the season-ending ATP finals in Turin in November.
He has won two titles this year and reached the top 10 for the first time, but is currently 11th in the Race to Turin rankings with a top-eight finish needed to qualify.
Norrie is the third seeded player to withdraw from the Korea Open after sixth seed Borna Coric dropped out on Monday and third seed Taylor Fritz withdrew on Thursday.
His next tournament is set to be the Japan Open in Tokyo, which starts on 3 October.
Top seed and world number two Casper Ruud, of Norway, plays Japan's Yoshihito Nishioka in the quarter-finals on Friday.
Brooksby, who is eighth seed, will now play Canada's Denis Shapovalov in the semi-finals on Saturday.