Cameron Norrie was forced to pull out of the Japan Open after testing positive for Covid-19 while out competing in South Korea.
Norrie wrote in a social media post: "Unfortunately, I had a positive Covid test in Korea and wasn't able to compete in the rest of my matches. I have no symptoms and I am feeling 100% fine.
"I was really hoping to go to Tokyo but because of the quarantine period I will not be able to play there."
Frances Tiafoe, the No. 4 seed, defeated Japan's Yasutaka Uchiyama in straight sets to reach the second round of the Japan Open on Monday.
American Tiafoe won 6-3, 6-4, in one hour and 14 minutes to set up a tie with Bernabe Zapata Miralles, who beat Taro Daniel 7-5, 7-5.
Elsewhere, Brandon Nakashima continued his good form to down Shintaro Mochizuki 6-3, 6-2.
Nakashima, who won the San Diego Open last month, won his sixth straight match in ATP competitions in just over an hour.
Pedro Martinez also picked up straight-sets victory, defeating Alexei Popyrin 7-5, 6-4.
Martinez will face either Casper Ruud or Jaume Munar in the second round, while Britain's Dan Evans faces Radu Albot on Tuesday.
Information from Associated Press contributed to this report.