Organisers said Zverev did not consult with the Roland Garros medical service before his match against Sinner.
Zverev said he had been ill since his third-round win against Italian Marco Cecchinato on Friday and had a body temperature of 38 degrees Celsius.
“It was nothing wrong but I am completely sick after the match with Cecchinato in the night. Yeah, what can I say? I’m completely sick,” said a masked Zverev during his virtual post-match news conference.
“I can’t really breathe, as you can hear by my voice. I had fever, you know, as well. It was 38. It was 38 in the night or in the evening.”
French government guidelines state that a person with COVID-19 symptoms – a body temperature above 38 degrees Celsius, a cough, sore throat, headache, muscle ache, loss of smell or taste – needs to isolate for seven days and immediately make an appointment for a test.
Zverev had declined to answer when he was last tested for the coronavirus at his post-match news conference.
The 23-year-old Zverev also played in Novak Djokovic’s Adria Tour event in the Balkans where multiple players including the Serbian world No.1 contracted the virus. Zverev said he had tested negative.
He drew criticism from Australian star Nick Kyrgios after being filmed partying just days after leaving the Adria Tour and promising to follow quarantine guidelines.
“To be honest, I warmed up today. I shouldn’t have played,” Zverev said after losing 6-3, 6-3, 4-6, 6-3 to Sinner.
Asked if he was concerned that he might get sick, 19-year-old Italian Sinner said: “He’s not positive or whatever. We have got tested quite many times, and obviously we were not that nearby.
“It’s like we always had the distance. I don’t think that I will have fever in the next days, or I hope so.”
Tournament organisers said that two players in the junior girls’ tournament had tested positive for COVID-19 and had been removed from the draw.
In total, approximately 3000 tests have been carried out since September 17, organisers said.