Why a tennis players' association has sued the groups that run the sport - and what they want

Two men's tennis players sit on the sideline holding drinks.

A class-action antitrust lawsuit filed by the Professional Tennis Players' Association in federal court in New York on Tuesday, calling the groups in charge of the sport — the women's (WTA) and men's (ATP) tours, the International Tennis Federation (ITF) and the agency that oversees anti-doping and anti-corruption efforts (ITIA) — a "cartel."