The Monte Carlo claycourt tennis tournament sued the ATP, stating that dropping the event from the Masters Series would violate antitrust laws.
The German Tennis Federation filed a similar suit in the same court last month, trying to block the ATP from downgrading the Hamburg Masters.
The Masters Series is the second tier of men's tennis tournaments, below the four major championships.
The Monte Carlo tournament, first held in 1897, has been a part of the Masters Series since that level began in 2000. It also was part of the Masters Series' predecessor, the Super 9 category of events.
"By downgrading it, we would really be put in a position where we wouldn't matter anymore," Monte Carlo tournament director Zeljko Franulovic said in a telephone interview. "We are talking about survival here, not only being a little less important event. We would probably disappear if we were downgraded."
The Monte Carlo Masters (April 15-22) and Hamburg Masters (May 14-20) are played on clay in the weeks before the French Open, the only Grand Slam played on the surface. The lawsuits seek to prevent the ATP from taking away those tournaments' Masters Series status starting in 2009.
An ATP spokesman issued a statement via e-mail on Monday: "The ATP has not received a copy of any lawsuit from the Monte Carlo tournament and therefore can't make any comment until a complaint is received and the ATP has had a chance to review it."
The ATP's corporate home is in Delaware.