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 practise 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.
Fritz said he had only been well enough to exercise in his room three days before he left quarantine.
But he made short work of the first set against world number 109 Duckworth and closed out the match convincingly despite a difficult second set.
"I felt, for whatever reason, confidence that I was going to be able to come out of the lockdown and strike the ball well," said Fritz.
"I was more concerned about the cardio and my condition after being really sick and not being able to do anything."
Shapovalov beats Johnson, Nishioka defeated by Kecmanovic
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.
"I was playing good tennis but then I started to get tired and I think the fact I couldn't close out the match in the second set was the reason I lost," said Nishioka.
In a second-round match-up earlier in the day, South Korea's Kwon Soonwoo beat American Mackenzie McDonald 6-3, 6-7 (4/7), 6-2.
You can follow all the action from the Japan Open on Flashscore.
