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Red Bull team principal Christian Horner has revealed that Sebastian Vettel considered quitting Formula One after changes in the regulations saw him suffer a downturn in form.

Following four consecutive world championships, Vettel struggled to fifth in the drivers' standings in 2014 and failed to win a race. He struggled to adapt to F1's new regulations, which included a new engine and a significant drop in the amount of downforce, while team-mate Daniel Ricciardo had a breakthrough year.

Vettel left Red Bull for Ferrari over the winter and his old boss Horner has given some insight into the four-time champion's struggle last year.

"Seb didn't enjoy the regulation changes," Horner was quoted by the Guardian . "He didn't enjoy the new engine, the feel from the new system, the power unit, the brake by wire, the lack of downforce. You could tell he wasn't happy. He was preoccupied and to compound that his team-mate [Daniel Ricciardo] won three races. There was that feeling 'am I enjoying this as much as I thought I was?'

"It was like someone had taken his toy away. It took him a while to get to grips with that. It was not something he was used to. He went through a period of disillusionment about the direction Formula One was going in. There was a stage last year when he thought whether he wanted to stop or not, whether he was getting the same level of enjoyment or not and whether or not he wanted to continue.

"He was just unhappy with the direction Formula One had gone. His previous four years had been so successful for him in a car he loved driving - and then suddenly things were very different. It raised some questions he had to deal with. He went back to basics and drove a kart in the middle of the year to get back to the bare essence of why he was a grand prix driver and rediscovered his passion for being a grand prix driver.

"We saw a little bit of it at the start if 2012 when the regulations were changed about the exhaust-blown diffuser, Horner said. "At the beginning of that year he won in Bahrain but it took him to the flyaways at the end of the year to win his second race and it was only when we found some performance around the rear of the car that it opened up his envelope for him."

But Horner said there were no hard feelings about Vettel leaving for Ferrari.

"His boyhood hero was Michael [Schumacher] and of course there was the lure of Ferrari. For any driver - the brand, the history, the mystique, is immensely powerful. And I think Sebastian felt the timing was right in his career. He needed that stimulus of a new challenge."

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