Alexander Zverev said he "should not have played" after his French Open last 16 loss on Sunday to Italian teenager Jannik Sinner and complained of feeling "completely sick".
The sixth seed summoned the doctor to ask for a cold spray during the first set of a 6-3, 6-3, 4-6, 6-3 loss and later claimed he had a fever.
Zverev, 23, was up to date with his tournament-mandated Covid-19 tests, the French tennis federation (FFT) said in a statement. His last one was on September 29.
However, the FFT added that the German "did not consult the tournament doctors before his match".
The US Open runner-up raised questions over whether he should have been allowed to play when he told reporters he had a temperature of 38 degrees Celsius (100.4 Fahrenheit) following his win over Marco Cecchinato on Friday.
"I am completely sick after the match with Cecchinato in the night. I can't really breathe, as you can hear by my voice," Zverev said after his Roland Garros exit.
"I had fever, you know, as well. Yeah, I'm not in the best physical state," said Zverev, who made 47 unforced errors and just 20 winners.
"To be honest, I warmed up today. I shouldn't have played. But I was hoping maybe for a three-set win or something like that."
Zverev was criticised in the summer after he was spotted partying despite vowing to self-isolate following Novak Djokovic's Adria Tour, during which a number of players tested positive for coronavirus.
The FFT said Zverev's previous coronavirus test was negative. The result was received on September 30.
"Today (Sunday) he received a reminder for his next test, to be carried out within five days of the previous results," the FFT added.
Youngest since Djokovic
Sinner became the youngest men's Grand Slam quarter-finalist since Novak Djokovic in Paris in 2006.
Sinner, aged 19 years and 56 days, set up a showdown with 12-time champion Rafael Nadal.
He is first player to reach the last eight on his Roland Garros debut since Nadal lifted the trophy in 2005.
Asked whether he was concerned for his own health after defeating Zverev at a tournament delayed four months due to the coronavirus pandemic, Sinner largely played down concerns.
"I didn't realise anything. I mean, I think in the first set he asked for the doctor on court, but I didn't ask for what," said Sinner.
"I was focusing about myself. So if he had something, yeah, I'm sorry for him, for sure."
"He's not positive for (Covid-19). We have got tested quite many times, you know, and obviously we were not that nearby," continued Sinner.
"It's like we always had the distance. I don't think that I will have fever next days, or I hope so. Maybe I have. I mean, you never know about that."
Sinner, last year's NextGen champion, dropped his first set of the tournament against US Open runner-up Zverev but demonstrated his resolve to close out the win in four.
"Today was very tough knowing that it's going to be a long match but in the end I'm very happy about my performance," said the 75th-ranked Italian.
"If you lose one set you just try and keep going. Even in the third set I knew I was still playing well," added Sinner, the second youngest player to begin this year's main draw.
"I tried to be focused in the first service game in the fourth set, which I did, and then it went quite well."