Players have frequently complained they do not receive a high enough percentage of the revenue generated by the sport, especially the four Grand Slams.

It was the driving force behind Djokovic's desire to form the PTPA.

The Grand Slams tried to develop the concept of a Premium Tour - featuring a streamlined season and greater financial rewards - but have so far found too many obstacles in their path.

Pospisil says the lawsuit is about "fairness, safety and basic human dignity".

"I'm one of the more fortunate players and I've still had to sleep in my car when traveling to matches early on in my career," he added.

"Imagine an NFL player being told that he had to sleep in his car at an away game. It's absurd and would never happen. No other major sport treats its athletes this way."

"It is time for free-market forces to enter professional tennis," said Drew Tulumello of Weil, Gotshal & Manges, the law firm instructed by the PTPA.

The PTPA looks enviously at the rewards earned by players in team sports such as football, NFL, baseball and basketball - and also the more comparable sport of golf.

Many of those who joined the breakaway LIV Tour now enjoy even greater wealth, but have lost the ranking points which facilitate entry into the major championships.

Saudi Arabia's intervention caused much bitterness, but changed golf dramatically.

The PTPA's methods are very different, but could yet have a similar effect.