Golf star Rory McIlroy signed a sports management contract at an informal meeting on the day of his agent's office Christmas party, court papers claim.
The former world number one is suing Dublin-based Horizon Sports Management and two other linked companies, the Malta-based Gurteen and Canovan Management, also based in Dublin, to try to recover some of 6.8 million US dollars (five million euro) he has paid in fees.
The 24-year-old Monaco resident claims he was exploited, misled and taken advantage of when he first signed a contract with Conor Ridge's Horizon at the end of 2011 and joined the stable of fellow countryman and major winner Graeme McDowell.
In court papers, McIlroy claims he had no legal advice or knowledge of negotiating before he signed up, and trusted them to charge the appropriate rates.
He claims Horizon charged almost four times what top ten golfers pay to agents.
The pre-tax rates were set at 5% of prize winnings and 20% of sponsorship and appearances money, a charge his lawyers claim is reserved for an "inexperienced or unproven golfer".
McIlroy, a Unicef ambassador, is also in dispute with Mr Ridge's firm over a 166,000 euro donation to the charity on the eve of a trip to Haiti.
The golfer, who first visited the disaster-hit country in 2011, pulled out of a return trip in March this year citing playing commitments but court documents claim the donation was reversed.
Horizon claim the money was paid "precisely for the purpose of protecting (McIlroy) and his reputation".
McIlroy's career soared in the year after he signed for Mr Ridge earning more than 11.7 million euro in 2012 before hitting a slump this year and earning about 2.35 million euro in prizes.
Early this year his game hit a low when he walked off the course during the second round in his defence of the Honda Classic and as he strode to the car park said he was "in a bad place mentally".
Horizon later put out a statement blaming pain from a wisdom tooth.
McIlroy maintains he did not get a draft of the contract before he was asked to sign in a lawyer's office on December 21 2011 - the day of Horizon's Christmas party and "in circumstances of great informality".
Horizon, Gurteen and Canovan are counter-suing McIlroy for at least 2.4 million dollars (1.9 million euro). The case will be heard in Dublin next year.
"The claim will be defended vigorously and comprehensively in legal process," Horizon said in a statement.
McIlroy also criticises a new contract he signed in March 2013 - when he secured his 100 million euro Nike deal - including amendments which meant Horizon could make money from the endorsement even if they did not represent him in the future.
The 24-year-old, from Holywood in Co Down, Northern Ireland, spilt from Mr Ridge in recent weeks and set up his own management operation, Rory McIlroy Inc (RMI).
He took Donal Casey with him after he played a key role in the Nike contract talks who it is claimed also had a falling out with Mr Ridge.
There was also a row over Horizon using complimentary airline seats booked by tournament organisers in McIlroy's name.
McIlroy wants both contracts rescinded and some of the five million euro in fees returned.
In his statement of claim, he alleges Horizon boss agent Conor Ridge acted primarily in the interests of his company.
"In particular Horizon and Mr Ridge were primarily concerned with maximising their own share of any commission," the court papers allege.
McIlroy claims Horizon breached its contract by:
:: using a first class flight to Abu Dhabi paid for by McIlroy for Horizon personnel to travel business class.
:: not telling him about a dispute with Horizon's head of strategy Donal Casey over the Nike contract.
:: costing him a contract with Creative Artists Agency (CAA), which represents David Beckham and Cristiano Ronaldo, because of Mr Ridge's unreasonable conduct.
:: failing to keep him fully informed on court action taken by Oakley against him which has cost him 1.5 million dollars (1.1 million euro) in legal fees.
:: he maintains Horizon should have been charging 0% for on-course earnings and 5-7% off-course.
The golfer's legal team applied to have the case heard in the Commercial Court in Dublin, a division of the High Court designed to fast track business cases.
Mr Justice Peter Kelly ruled the golfer's claim, and the counter claim, both exceeded the one million euro threshold to be heard in his court.
It is due to be listed for trial in October 2014 to fit in with McIlroy's extensive travelling worldwide.
McIlroy, a two-times Major winner, began a month-long tour of Asia today, well away from the confines of a crowded courtroom and amid reports that his relationship with tennis star Caroline Wozniacki had ended.
His public relations advisers have said they will not comment on the star's private life.
Despite the reports, Wozniacki's Twitter page still carried a profile of the couple in happy times.
McIlroy, whose address is listed in court papers as Avenue Princess Grace, Monaco, spent last week in Ireland staying mainly in a top Dublin hotel.
He played a round of golf with former US president Bill Clinton at Portmarnock and enjoyed a dinner with golf fanatic U2 star Bono one night. He also watched Ulster play in the Heineken Cup in Belfast on Friday night.
Horizon will serve its counter claim on November 26.
McIlroy's new business is headed by ex-Horizon actuary Mr Casey while "long-term family friend" and business executive Barry Funston will run the golfer's foundation. Both men will serve on the group's board, along with McIlroy's father, Gerry.
The golfer previously split from Chubby Chandler's International Sports Management, which had represented him since he became professional in 2007, for Horizon, four months after winning his first major at the US Open.