Ferrari has "seriously picked up its game" ahead of the 2016 Formula 1 season and will provide a genuine threat to Mercedes, says Lewis Hamilton.
The Scuderia emerged as Mercedes' closest challenger in 2015 and demonstrated strong pace in pre-season testing, topping the times on five of the eight days.
"Honestly we are going to have a real battle," Hamilton told reporters at Mercedes' pre-season launch in Stuttgart.
"Ferrari has seriously picked up its game. That's my opinion.
"It's still difficult to know. The car is good but Ferrari was strong towards the end of last year and they have made a significant improvement from last year to this year.
"They did some incredible times at the last test in some of their long runs and I believe they didn't bring an upgrade to any of the test days, so I imagine they will bring a good package to the first race.
"You are going to see a good fight, which is what I want and we want in the sport. I'm excited about that."
Nico Rosberg said last week he does not know if Mercedes will be ahead when the season starts in Melbourne and echoed those thoughts on Friday.
"We don't know the fuel levels, but we can estimate and it's looking extremely close," he said. "We need to be careful."
Mercedes motorsport chief Toto Wolff would not be drawn on where he believes the team is in the pecking order but said people should be wary of drawing conclusions from testing.
"We achieved all the mileage targets we had hoped for and performance-wise where we should be among the top teams," he said.
"Ferrari looked equally good and there were some interesting laps in their long-run simulation and also on the single laps that give us an indication their car is very fast as well.
"But it's like the famous saying, on Sunday when the flag drops the bullshit stops.
"By then we will know whether we are good enough for another championship and good enough for race victories or not."