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 No.1 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.