World number one Novak Djokovic dealt a further blow to the hopes of the US Open going ahead after claiming it would be "impossible" to play under the "extreme" proposed safety protocols.
The hard-court grand slam is due to begin at Flushing Meadows in New York on August 31, but there are serious doubts over the tournament due to the ongoing coronavirus crisis.
World number two Rafael Nadal said earlier this week that he is not currently prepared to travel abroad for tournaments due to global health fears and quarantine restrictions.
And now Djokovic, a three-time winner of the tournament, has said the safety restrictions imposed, including a ban on travel to Manhattan and only being allowed one person with him, are "extreme".
"I had a telephone conversation with the leaders of world tennis, there were talks about the continuation of the season, mostly about the US Open due in late August, but it is not known whether it will be held," the Serbian told Prva TV television.
"The rules that they told us that we would have to respect to be there, to play at all, they are extreme.
"We would not have access to Manhattan, we would have to sleep in hotels at the airport, to be tested twice or three times per week.
"Also, we could bring one person to the club which is really impossible.
"I mean, you need your coach, then a fitness trainer, then a physiotherapist."
The men's ATP Tour and the women's WTA Tour are currently suspended until August 1, with some tournaments coming after that already cancelled, though exhibition events have started this week, with world number three Karolina Pliskova playing in the LiveScore Cup in Prague.