Mercedes' non-executive chairman Niki Lauda fears that 2017 could be the year that Mercedes are ousted from the top of Formula 1.
Lauda was reacting to what he'd seen in eight days of pre-season testing at the Circuit de Barcelona-Catalunya.
Ferrari stole the show, with Kimi Raikkonen setting the best time of anyone. The team also demonstrated good reliability and were nearly a match for Mercedes in terms of mileage.
Lauda said that Mercedes were going to struggle to stay ahead of Ferrari in 2017 and retain their twin crowns.
"Everyone is saying that if you look at the past, Mercedes will win again," Lauda told the German broadcaster RTL.
"But these are naive people. I think we have a tough year ahead of us," he added.
"At the moment, no one can say who will be in front in Melbourne. But the field is definitely closer together."
Lewis Hamilton said on Friday that he'd not been holding anything back in Spain last week. "The time you saw was the time we could do. I didn't feel when I got out that I could have gone faster."
But he added that he thought Ferrari had more to come than they were willing to show in pre-season testing
"Sometimes there have been potentially some teams sandbagging in the past," he said. "In testing the goal is not necessarily to go as quick as you can, it's to find out what you have in the car.
"It looks very positive for Ferrari," he added. "I don't think that's a bluff."