World number three Stefanos Tsitsipas said he would only get the Covid-19 vaccine if it became mandatory to compete in tennis.
While the men's ATP Tour has publicly encouraged players to get vaccinated, the 23-year-old Greek is among those who still have reservations.
"No one has told me anything. No one has made it a mandatory thing to be vaccinated," he told reporters, when asked if he would seek a vaccine while competing in the U.S.
"At some point I will have to, I'm pretty sure about it, but so far it hasn't been mandatory to compete, so I haven't done it, no," added Tsitsipas, who received a first-round bye in the Masters 1000 tournament in Cincinnati.
He reached the French Open final in June but suffered a shock, first-round exit at Wimbledon, where he told reporters he found it challenging to live and compete in the Covid-19 "bubble."
The Covid-19 vaccine has divided opinion within tennis.
World number one Novak Djokovic said in April he hoped the Covid-19 vaccine would not become mandatory for players to compete and has declined to answer questions regarding his own vaccination status.
However, fellow 20-time Grand Slam winners Roger Federer and Rafa Nadal feel athletes need to play their part to get life back to some form of normality.
Federer said in May that he received the Pfizer-BioNTech vaccine, while Nadal said: "The only way out of this nightmare is vaccination. Our responsibility as human beings is to accept it.
"I know there is a percentage of people who will suffer from side effects, but the effects of the virus are worse.”
Spectators will not be allowed to attend qualifying rounds at this month's U.S. Open due to the Covid-19 pandemic, the United States Tennis Association (USTA) said last week.
The USTA previously said it would allow full fan capacity for the main part of the tournament.