Lewis Hamilton believes Ferrari has "something up their sleeve" going into the Formula 1 season-opening Australian Grand Prix this weekend.
In pre-season testing, while Mercedes dominated in terms of reliability, Ferrari emerged quickest on five of the eight days at the Circuit de Catalunya in Barcelona.
The suggestion has been Mercedes was sandbagging, but reigning champion Hamilton said: "This year the whole pack looks like it's a little closer.
"But I think they [Ferrari] have something up their sleeve, personally, this weekend.
"The Ferraris, they are going to be a lot closer than they are talking."
Asked whether it was a challenge he welcomed, Hamilton said: "Absolutely."
Sebastian Vettel, who in 2015 was the only other driver to win races aside from Hamilton and team-mate Nico Rosberg, feels Ferrari can give Mercedes a better run for its money this year.
Asked whether Ferrari has what it takes to challenge this year, Vettel said: "I think we have. The question is whether we are ready in time.
"We are on a good track, we are making progress, we did so last year.
"We have made a step, whether it is big enough, we will have to see.
"Going into this season, I do not think it is a surprise to say Mercedes is still favourite, but we will try to be the favourite in the future."
Throughout testing Mercedes' focus was on mileage on medium tyres, proving reliability, with the quicker soft tyres barely touched.
Hamilton feels the plan could have been different as he would have preferred more performance running to get a better feel of the car with regard to its pace.
"Performance runs are always the most fun," he added.
"The long runs are...long, unexciting, but we did what we had to do.
"We went into testing with a goal of achieving 800 kilometres per day. We didn't actually think we would do that.
"Perhaps with hindsight we would have shifted our running, but at the end of the day it suited us perfectly."