Organisers of the Monte Carlo Masters tournament are suing the ATP Tour over plans to downgrade the event's status.
The anti-trust lawsuit follows a similar move by organisers of the Hamburg Masters.
It marks the latest in a series of objections to a restructuring plan being pushed through by the ATP's chairman Etienne de Villiers.
De Villiers wants to restructure the tour to ensure the world's top players play in the most prestigious events
The Masters Series is the second tier of men's tennis tournaments, below the four major championships.
Madame de Massy, the president of the Monte Carlo tournament committee said: "We are calling upon the ATP's management to listen to the message that the tennis world is sending: stop this plan before it causes real damage to tennis."
Monte Carlo Masters Series tournament director Zeljko Franulovic added: "Today marks a sad day for tennis when we are forced to take legal action against our own representative body."
It is understood around 60 players, including world number one Roger Federer, signed a petition last week against changes to reduce European Masters spring events from three to two.
The third event of the three is the Rome Masters, and all are played on clay in the build-up to the French Open.