World No.3 Tsitsipas revealed he has not yet been jabbed against COVID-19 with the tennis world divided on the issue.
The ATP Tour has publicly encouraged players to get vaccinated, but Novak Djokovic is among the high-profile players to have stated it should not be compulsory to compete.
When asked ahead of the Western and Southern Open if he would get the vaccine, Tsitsipas said he still had reservations.
"No one has told me anything. No one has made it a mandatory thing to be vaccinated," the Greek said.
"At some point I may 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."
Tsitsipas received a bye through to the second round in Cincinnati, where he will play either Sebastian Korda or Laslo Dere.
The 23-year-old, who has a tour-high 45 wins this season, climbed up to No.3 in the world rankings last week and boldly targeted overhauling Djokovic in top spot.
"The rankings are there for a reason. They signify something important," he added.
"I think that the very next step would be the number one spot, which I hope I can get to one day.”