Novak Djokovic and Vasek Pospisil have confirmed the formation of a breakaway players union, the Professional Tennis Players Association (PTPA).
World number one Djokovic and Pospisil each posted a picture of the first players to sign up for the PTPA on court together at Flushing Meadows on Saturday.
Djokovic resigned as president of the ATP Player Council to front the rival union and attempt to increase player power.
The tweets from Djokovic and Pospisil read: "After yesterday's successful meeting, we are excited to announce the beginning of the Professional Tennis Players Association (PTPA). The first player only association in tennis since 1972."
However, the new male-only association has some high-profile critics.
Andy Murray believes it would send a more powerful message were the WTA to be brought on board, while Roger Federer and Rafael Nadal have each come out against the PTPA.
Nadal wrote on Twitter: "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.
"These are moments where big things can be achieved as long as the world of tennis is united. We all, players, tournaments and governing bodies have to work together. We have a bigger problem and separation and disunion is definitely not the solution."
Federer added: "I agree @RafaelNadal. 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."