Vasek Posposil has labelled the ATP boss a "f***ing a**hole" in an explosive on-court meltdown at the Miami Open.
The Canadian crashed out of the Masters 1000 event on Wednesday, losing 3-6, 6-4, 3-6 to American Mackenzie McDonald.
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The World No.67 went on a wild tirade towards the end of the first set after launching a ball out of the stadium and smashing a racquet.
He received a code violation for the racquet smash before being deducted a point for verbal abuse to hand the set to McDonald.
He then went on an extraordinary expletive-laden tirade to chair umpire Arnaud Gabas about ATP Chairman Andrea Gaudenzi.
"An hour and a half the chair of the ATP f***ing screaming at me in a player meeting for trying to unite the players," Pospisil said to Gabas.
"For an an hour and a half ... The leader of the ATP... F***ing a***hole.
"If you want to default me, I'll gladly sue this whole organisation."
Gabas seemed gobsmacked by the claims and told Pospisil he should take up his grievances with Gaudenzi off the court.
The Canadian regained his composure to take the second set, but eventually fell in the third.
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Pospisil was formerly on the ATP Player's Council and has united with Novak Djokovic in recent years in trying to advocate for their fellow players.
“Tennis needs major change,” he said last year.
“It wasn’t until I was in my mid-20s until I opened my eyes to how many issues there were in tennis.
"I felt like ‘Why aren’t we able to change anything as players'.
"We have so much leverage if we actually decided to get organised and use it, because without the players there is no tennis tour."
Djokovic sent shockwaves through the sport last September when he stepped down as head of the ATP council along with Pospisil, John Isner and Sam Querrey and announced the formation of the Professional Tennis Players Association (PTPA).
Djokovic has described the PTPA, which has already attracted the support of more than 150 players, as a platform for the views of the athletes that can co-exist with the ATP.
Tennis governing bodies have, however, opposed the move and called for unity.
Player council members Roger Federer and Rafael Nadal have echoed those sentiments, while some other top names in men's tennis have asked for more clarity.
"From the council standpoint, especially the timing, the communication, the way this came to be, it's not something we had discussed or communicated so I feel like we were blindsided by it," Kevin Anderson said.
"I don't agree with how they're going about it and the method.
"I will always respect their effort to help the players, I think that's a great thing. But that's what we've done in the council and what we continue to do."
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