Four-time Formula One world champion Sebastian Vettel considered quitting as he struggled with the sport's new direction last year, according to the German's former Red Bull boss Christian Horner.
While teammate Daniel Ricciardo won three races, Vettel, who had been champion for four years in a row and won 13 grands prix in 2013 including nine consecutively, ended the campaign empty handed.
"Seb didn't enjoy the regulation changes," Horner told British reporters ahead of this weekend's first pre-season test in Jerez, Spain.
"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 teammate won three races. There was that feeling 'Am I enjoying this as much as I thought I was?'"
The German has since left Red Bull and joined Ferrari, who also failed to win a race last year for the first time since 1993.
"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."
Horner said it would take time to get used to seeing Vettel in Ferrari's red overalls but felt the German was right to seek a change and a new stimulus.