British number one Cameron Norrie pulled out of the Korea Open less than an hour before 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.
Compatriot Liam Broady, 28, withdrew from the Tel Aviv Open before his quarter-final against second seed Croatian Marin Cilic.
Broady, the British number five, beat Dutch fifth seed Botic van de Zandschulp in three sets to reach the second ATP quarter-final of his career and first since 2017.
No reason has yet been given for his withdrawal.
Serbian top seed Novak Djokovic will play Canada’s Vasek Pospisil on Friday in the pick of the other quarter-finals.
Norrie’s ATP Finals hopes hit
Norrie has won two titles this year and reached the top 10 for the first time.
He is eighth in the world and chasing a place at the season-ending ATP finals in Turin in November.
Norrie had earmarked the Korea Open and Japan Open, which starts on 3 October, as an opportunity to pick up points in the Race to Turin rankings. Currently 11th, he needs to finish in the top eight to qualify.
He became 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.
Top seed Casper Ruud, of Norway, is also out after losing 6-2 3-6 6-2 to Japan’s Yoshihito Nishioka in the quarter-finals on Friday.
Rafael Nadal will move above Ruud in the world rankings as a result of the defeat, with the US Open and French Open runner-up falling from second to third.
Nishioka will play United States’ Aleksander Kovacevic in the last four, while eighth seed Brooksby will play Canada’s Denis Shapovalov in the other semi-final.
