Mercedes executive director Toto Wolff has said that Williams and Red Bull will be his team's biggest rivals next year.
Mercedes dominated in 2014 and won all but three of the races, as drivers Lewis Hamilton and Nico Rosberg scrapped for the Drivers' Championship, but Wolff isn't expecting a similar season in 2015.
When asked who would be their most dangerous opponent, he told reporters: "I would put Williams at the top of the list. But I also fear Red Bull. They won four titles in a row and put every resource into fighting for the world championship again."
Red Bull have a new driver lineup after Sebastian Vettel left the team to join Ferrari, with young driver Daniil Kvyat joining Daniel Ricciardo, while Wolff also singled out Fernando Alonso as a threat at McLaren.
Wolff's wife, Susie, is the Williams test driver.
