Ferrari are not the only rival team Mercedes need to worry about this season, according to Nico Rosberg, who has been impressed by Red Bull's form.
Mercedes have continued their domination of Formula One with Rosberg claiming victories in the opening three races of the 2016 campaign, while Lewis Hamilton has sealed two podiums.
Their lead in the constructors' championship is already 53 points to Ferrari, while Red Bull are a further four back.
While many expect Sebastian Vettel and Kimi Raikkonen to be Mercedes' main threat this year, Rosberg believes Red Bull's performance at the Chinese Grand Prix - where they finished third and fourth - shows they are also in contention.
"Red Bull was a surprise," the German is quoted as saying by Autosport.
"They were close in qualifying and close in the race, although we do see Ferrari [as being] right on us whereas Red Bull were still about half a second away [in qualifying], so that's still a fair chunk.
"But they did make a big step, which was a surprise considering they likely don't have as much engine power as us, so they are a force to be reckoned with now, for sure."
Ferrari's challenge has been hampered by early reliability problems - Vettel failing to start in Bahrain due to a blown engine and Raikkonen retiring in Australia.
Rosberg added: "Ferrari really hasn't shown what they can do at all yet.
"They've had so many mishaps all the time, and we really think they're very close to us.
"In qualifying, Kimi messed up otherwise he would've been on it, close to us, so we need to keep our heads down and keep pushing."