Red Bull team boss Christian Horner believes the exertions of the past few seasons, among other things, have taken its toll on Sebastian Vettel.
Having won four consecutive World Championships, Vettel was expected to once again be challenging for top honours this campaign, but he has been outshone by new team-mate Daniel Ricciardo.
Horner says battling for the title for several years has played a role while he feels the new regulations have also made the 27-year-old's life difficult.
"It is a combination of several things," he told Germany's Auto Bild Motorsport. "First, when you have fought for the title for five years, it does wear you out a little bit, but that is not the fundamental problem.
"The way Sebastian brought out those extra tenths from the car in recent years was quite unique. He is very sensitive to the behaviour of the car, especially when braking."
Horner admitted that the German has "lost part of his feeling for the car" due to the mandatory 'brake-by-wire' systems this year.
He added: "The driveability was really bad, so Seb could not look after the tyres in the way that he always has done."
Ricciardo has edged Vettel 6-5 in qualifying but he leads 9-2 in race results with the Australian claiming two race victories as well. Vettel, though, has also suffered three retirements through no fault of his own.
"His pace in Hungary showed he is getting the feeling for the car back again," said Horner.
"And we can't forget how many mechanical problems Sebastian has had - many of them just little things that have disrupted his flow. So he has had less time to adapt his driving style."