Roger Federer, Rafael Nadal and other members of the ATP Player Council have questioned the proposal by Novak Djokovic and Vasek Pospisil to form a breakaway body to represent male players only.
The issue brings the sport's politics into the spotlight just days before the US Open
Also against the proposal: the ATP men's tour and the sport's other governing bodies, including the WTA women's tour and the four grand slam tournaments.
They issued a joint statement on Saturday saying: "It is a time for even greater collaboration, not division."
World No.1 Djokovic was, however, still determined to push ahead with the Professional Tennis Players Association (PTPA).
"I have read in the letter from ATP, that they think that ATP cannot co-exist with the association," the 17-times grand slam winner said.
"I have to respectfully disagree.
"This is not a union. This is a player association."
In an email sent to players earlier in the week, Djokovic and Pospisil - who along with John Isner have resigned their roles on the ATP Player Council - said they would be the initial co-presidents of the group, which would also be led by up to nine trustees - all players.
"We are not calling for boycotts. We are not forming parallel tours," Djokovic said after winning the Western & Southern Open on Saturday, two days before play begins at the US Open.
"Of course I would love to have Roger and Rafa on board. Of course I would love to have all the players on board. But I understand.
"I truly understand that some of them have different opinions and they don't think the time is right."
Federer and Nadal joined player council representatives Kevin Anderson, Jurgen Melzer, Sam Querrey and Bruno Soares in sending a note to players raising questions about the proposal and saying they're against it.
"The world is living a difficult and complicated situation. I personally believe these are times to be calm and work all of us together in the same direction. It is time for unity, not for separation," Nadal tweeted.
Federer quote-tweeted his on-court rival to say he agrees and added: "These are uncertain and challenging times, but I believe it's critical for us to stand united as players, and as a sport, to pave the best way forward."