Tennis fans 12 years or older will be required to show proof of a Covid-19 vaccine in order to attend the U.S. Open, tournament officials announced Friday, just three days before the main draw was set to begin.

The New York City mayor's office on Friday mandated proof of vaccine to enter Arthur Ashe Stadium, where the major puts on its primetime matches, the United States Tennis Association (USTA) said.

The USTA then elected to extend the vaccine requirement to all attendees 12 years old or older at the Billie Jean King National Tennis Center, regardless of which facilities ticketholders plan to access.

"Any U.S. Open attendee with tickets to Arthur Ashe Stadium, Louis Armstrong Stadium, the Grandstand, or the grounds of the U.S. Open, will be required to provide proof of at least one dose of a Covid-19 vaccine," the USTA said in a written statement.

The USTA confirmed that the mandate does not apply to athletes competing in the tournament, some of whom have previously expressed reluctance to receive the shot.

World number three Stefanos Tsitsipas said earlier this month he would only get the COVID-19 vaccine if it became mandatory to compete in tennis.

The announcement marked an abrupt change for the U.S. Open, the crown jewel of the American tennis calendar and the year's final major, which previously said it would rely on unvaccinated fans to wear masks.

The tournament operated without fans in 2020.