The Professional Tennis Players' Association (PTPA) is seeking a court order to prevent the ATP Tour engaging in "improper, coercive or threatening communications" with players.
The association alleges the ATP has tried to pressure players into signing pre-prepared statements saying they had no prior knowledge of the legal action launched this week by the PTPA.
The union, which was co-founded by 24-time major winner Novak Djokovic, cited "anti-competitive practices and a blatant disregard for player welfare" in lawsuits filed on Tuesday.
Djokovic is not one of the players listed as a plaintiff in the official court documents, but said publicly at the Miami Open on Thursday that there are elements of the lawsuit with which he agrees, and some with which he does not.