Toto Wolff is worried about Williams as a threat next season.
Toto Wolff has singled out Williams as the most dangerous opponent for title defenders Mercedes in 2015.
Mercedes utterly dominated the first year of the new turbo V6 era last season, with 2010-2013 world champions Red Bull in second place.
But it was Mercedes-powered Williams that ended the season as arguably the works team's closest challenger.
I also fear Red Bull
Asked by Italy's La Gazzetta dello Sport to name the "most dangerous opponent" for Mercedes in 2015, team boss Wolff answered: "I would put Williams at the top of the list.
"But I also fear Red Bull; they won four titles in a row and will put every resource into fighting for the world championship again," he added.
Next best in 2014 was the beleaguered F1 giant Ferrari, but Wolff refuses to write off the Maranello team despite the current slump and turmoil.
"I think it's normal that when there are changes of this type, there may be difficult times," he said.
Vettel at Ferrari is a threat
Wolff also said Sebastian Vettel's move to Ferrari must be taken seriously.
"Sebastian brings four world titles, enthusiasm and experience," he said. "It will be an incentive for the people of the incredible Ferrari.
"Never write them off. Ferrari is an icon and it's important that soon they are able to win again. I am convinced they will return to being competitive, and perhaps as early as 2015."
Wolff also told La Gazzetta that Fernando Alonso is a threat at the wheel of any car, even though McLaren's early tests with the new Honda engine were not successful.
"The problems they (Honda) had were not associated with the engine, but the electronics," he said.
"Difficulties of that type are unavoidable in the first tests -- we've all been through it," Wolff added.