TV mogul Simon Cowell has described his fallout with Cheryl Fernandez-Versini as a "nightmare" and admitted that "there was something not quite right" about US X Factor.
The duo's friendship came to an abrupt end when Cowell, 54, fired the singer from the US show just weeks into her high-profile job.
Cowell said that the judge, 31, who had "put on a bit of weight" and whose confidence had plummeted following the end of her marriage to Ashley Cole, had become unrecognisable.
He told Radio Times magazine: "My falling-out with Cheryl came soon after her divorce and was a nightmare. We were all wound up.
"There was so much expectation on the show in America and I definitely wasn't myself. I was uptight."
He said of the former Girls Aloud star's appearance, complete with big hair, purple trousers and an orange frilly top on the first day filming the US show: "When I saw her I didn't recognise her. Literally. The hair and outfit were crazy. Just totally crazy.
"I asked her manager, 'What's she wearing, what's happened to her hair?' He was defensive about it. I haven't a clue why she turned up like that."
He added: "She'd put on a bit of weight, but not loads. That wasn't the issue. The only way I can describe it is that it wasn't Cheryl sitting in that seat. She'd lost her confidence."
Cowell admitted of the US X Factor, which ended after three series and falling ratings: "In hindsight I don't think that first day worked anyway. There was something not quite right about that show in America."
He said that Fernandez-Versini - who is back as a judge on the UK X Factor - "turned on" him when he mentioned her outfit and told her that her first few days on the US show had not gone well.
She later told him that he had "betrayed" her.
"If I could go back in time I would have kept her on the show. It was stupid doing what I did. I didn't sack her personally. That was the second-biggest mistake I made. When you're under pressure you make stupid mistakes," he said.
Fernandez-Versini has previously revealed that she sent Cowell a "very abusive" text message telling him: "F**k you, f**k the orange and purple outfit, f**k big hair, f**k you all. In fact f**k off."
Cowell told the magazine: "The worst it got between us was when she sent me a text message.
"It was like when you get a really horrible letter from an ex-girlfriend. The general tone was something along the lines of 'F**k you, you've hurt me, you've let me down, I can never forgive you'. I shrank."
He labelled the X Factor judge "beautiful" and "a tough little nut"..... "but not really my type", adding that the singer, like himself, is a "killer" with "steel" in her eyes because she knows how to get what she wants.
Fernandez-Versini said of her sacking: "In hindsight he was right because I wasn't myself. He wasn't himself either. He was under a lot of strain because it was a new venture.
"But he handled it wrongly - he got somebody else to sack me. I didn't want to talk to him at the time. I didn't want to talk to anybody. The sacking came on top of everything else, it was just the cherry on the cake."
She said that the music mogul - who got someone else to sack her - found confrontation difficult because he is a "good person", adding: "I've warned him never to let me down again. Just let him try."
But in a dig at her boss, she added: "He's more ruthless than me but I'm stronger in my opinions; I believe what I'm saying."