Benoit Paire has claimed he was given permission by doctors to compete at the European Open in Hamburg despite twice testing positive for coronavirus.
The world number 25 retired from his opening round match with Casper Ruud on Wednesday when a set and break down.
He was competing in his second tournament since withdrawing from the US Open with a positive COVID-19 test in August, having also fallen at the first hurdle at last week's Internazionali d'Italia.
Speaking after his exit in Germany, Paire revealed he had been self-isolating in his hotel room in the lead up to the match and was only allowed to practice for one hour but was still given the green light to compete.
"I spent 10 days in a hotel room during the US Open and again now," he said. "I am tired. I practiced for one hour and then went to the hotel room - it is impossible to do. I was tired and had to stop.
Asked to clarify if he tested positive again during his time in Hamburg, Paire replied: "Yes, I was positive here in Hamburg since I arrived. The only negative test I got was yesterday.
"Before that, there was two in a row where I was positive. But the rules seem different here. The ATP has to explain what the rules are. In Paris right now, players are testing negative, but because the coach is positive, they cannot play.
"Here in Germany, you test positive and can play. So again thank you to the tournament and the doctor here in Hamburg for letting me play. There are some rules I obviously don't understand too much.
"I was negative after the US Open, negative in France, negative in Rome and then positive in Hamburg. That's all I know.
"It's tough for me to understand when I arrive here in Hamburg and they tell me to stay in my room again. I really don't understand. Honestly, it is not easy for me."
Paire is due to take part in the French Open, his home grand slam, which begins on Sunday.