Lewis Hamilton says Mercedes needs to be wary about its prospects for the season ahead, amid fears that rival Ferrari is closer than many people think.
Ferrari has made no secret of its world championship target for this season, and has heavily revamped its engine and car's aerodynamics for 2016.
Hamilton says that all his indications point to the team having made a 'significant' leap forward in form.
Speaking at a Mercedes event in Stuttgart on Friday about how his team stacks up against Ferrari, Hamilton said: "Honestly, I think we're going to have a real battle, Ferrari have seriously picked up their game - that's my feeling.
"It is still difficult to know. The car is good but Ferrari was strong at the end of last year. I think they've made a significant improvement from last year to this year.
"They did some incredible times in the last test on some of their long runs. I believe that they will bring an upgrade after the test days, so you can imagine they could bring a good package for the first race.
"So I think we're going to have a fight. You are going to see a good fight, which is what I want – and it is what we want. So I'm excited about that."
Although Mercedes had a faultless pre-season test programme and, on fuel and tyre corrected pace, was fastest overall, its Maranello rival did not appear to be that far behind.
Teammate Nico Rosberg also reckoned his team needs to be cautious.
"It's not possible to know for now, because we don't know the fuel levels," Rosberg added.
"But we can estimate and, for sure, it's looking extremely close. We need to be careful."