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 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 quarterfinals.
