British number one Cameron Norrie pulled out of the Korea Open less than an hour before his quarter-final against American Jenson Brooksby.
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.