Gareth Senior is passionate Leeds United fan who regarded Tony Dorigo as one of his heroes until the day he introduced him to his girlfriend, his solicitor told the court.
Senior described in court how his relationship with Ms Joss began to sour following a chance meeting with the former football professional at a music event in Manchester in August 2017.
Senior had left his partner of 25 years to start a relationship with Ms Joss in April 2016.
The defendant told the court he was big fan of Leeds United and Mr Dorigo in particular.
He described how he had taken Ms Joss to the August bank holiday event in Manchester as a birthday treat.
He said: "We were having a couple of drinks at an outdoor music event.
"It was very busy and the sun was shining and I saw someone who I thought was Mr Dorigo.
"I was surprised to see an ex-Leeds footballer, especially in Manchester."
Senior said Mr Dorigo stopped to speak to the couple.
He continued: "Claire informed me that October that Mr Dorigo messaged her to ask her to meet him when she was in London, as she often works there, or when he was over in Leeds. To meet one to one and not not with me present."
He told the court that his relationship broke down with Ms Joss in late November or early December.
Senior said he became suspicious after friends had told him Ms Joss had been seen with Mr Dorigo over the Christmas period.
He told the court he spotted Ms Joss meeting a man as he left work at Quarry House on February 6 and followed them into the shopping centre.
He said: "I stood outside John Lewis a bit dazed, upset perhaps and I saw Claire upstairs at the window.
"Mr Dorigo appeared in the window some seconds later."
Senior described how he went into The White Company to confront his rival.
He said: "He had his back to me and I wanted him to face me. I pushed him. That's the best of my recollection."
"My voice was raised and I said something along the lines of "what kind of man do you think you are?
"I was telling Mr Dorigo that he was pathetic because he was down in front of me holding his face."
Senior said he was punched twice by Ms Joss as she shouted at him to get away.
During cross-examination, prosecutor Gareth Henderson-Moore asked Senior if his account was incorrect.
He replied: "My emotions were strong but I am not a violent person and I have never been a violent person.
"I am ashamed and embarrassed of approaching Mr Dorigo in the way that I did. If it is possible I would like to offer my apologies to Mr Dorigo and to Claire.
He added: "But there is zero evidence of any headbutt."
Senior's barrister Stuart Field said: "If you were to have to ask him a couple of years ago to tell a little bit about himself he would say he was a happily married man.
"He has been an IT consultant for a lengthy period of time.
"Leeds United is his passion and among his heroes was Tony Dorigo.
"Through his work he met and fell in love with Claire Joss and was prepared to sacrifice all he had to set up a life with her and he did."
Mr Field said Senior would now be the subject of an investigation by the NHS and may lose his job.