Red Bull principal Christian Horner claims Sebastian Vettel almost quit Formula One after becoming frustrated by a host of rule changes.
After a period of domination in F1 that saw Vettel win four consecutive world titles between 2010 and 2013, the German endured a miserable 2014 season.
Vettel - who departed Red Bull for Ferrari at the end of the last campaign - failed to win a race and was outperformed by team-mate Daniel Ricciardo on his way to fifth in the drivers' standings.
"Seb didn't enjoy the regulation changes," Horner told 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 [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."
Vettel will join up with Kimi Raikkonen, the last Ferrari driver to win the world title back in 2007.