Investment banker Jeffrey Blue says Mr Ashley promised to pay him £15 million if he used his expertise to increase Sports Direct’s share price to £8 a share.
He says Newcastle United owner Mr Ashley paid only £1 million - and he wants £14 million damages.
Mr Ashley denies the claim and says Mr Blue is “trying to take me for £14 million off the back of some drunk banter”.
He is scheduled to start putting his side of the story on Wednesday.
A judge began analysing evidence at a hearing in London on Monday.
Mr Justice Leggatt heard the dispute between Mr Blue and Mr Ashley relates to a conversation in a central London pub in 2013.
Mr Blue says Mr Ashley made a promise during a meeting in the Horse and Groom four years ago.
The investment banker told the judge Mr Ashley is a “serious businessman”.
He said the work ethic at Sports Direct was “like nothing else I have ever seen”.
But he said Mr Ashley sometimes did business “in unorthodox ways and in unusual venues”.
Mr Blue has told how Mr Ashley vomited into a fireplace after a Sports Direct senior management meeting which was “effectively a pub lock-in”.
He said Mr Ashley would nap under tables at “boring” meetings.
Mr Ashley has told the judge, in a written statement: “I can’t believe that (Mr Blue) is now trying to take me for £14 million off the back of some drunk banter that he is seeking to engineer into something more.”