American Taylor Fritz said he surprised even himself after climbing out of his sickbed, flying to Tokyo and beating Australian James Duckworth at the Japan Open on Wednesday.
Fritz spent seven days in quarantine in a Seoul hotel room after getting "pretty sick" upon arriving last week for the Korea Open, which he was forced to withdraw from.
The 24-year-old left South Korea and flew to Japan on Wednesday morning and barely had time to practice before he was due on court for his first-round match against the unseeded Duckworth.
The number three seed still emerged a 6-2, 6-7 (2/7), 6-1 winner and said the match had been "better than I thought it would be".
"I don't feel amazing but I'm really happy that I was able to go through all that and still show up for my match," said Fritz.
Earlier in the day, Canadian number seven seed Denis Shapovalov beat American Steve Johnson 6-3, 7-6 (7/3).
Shapovalov lost to Japan's Yoshihito Nishioka in the final of last week's Korea Open and he said the Wednesday start to his Tokyo campaign had helped him recover.
"I think the most important thing for me is to feel mentally fresh," said the world number 22.
"I feel mentally very motivated, and when you do lose in a final it almost motivates you a bit more than winning, to play the next week."
Nishioka was looking to make a triumphant homecoming after winning his second ATP Tour title in Seoul but he lost 2-6, 7-6 (7/1), 6-2 to Serbia's Miomir Kecmanovic.