Some top tennis professionals and a group co-founded by former No. 1 men’s singles player Novak Djokovic and backed by hedge fund manager Bill Ackman are suing the sport’s governing bodies for allegedly running a cartel that caps prize money,

The PTPA is for all intents and purposes a union for tennis players, but it is not formally recognized because they are not classed as employees. It is part of the World Players Association, a global collective of the unions and associations that represent professional athletes across sport.