Pecking order unclear after Valencia test
Team figures including Ross Brawn, Norbert Haug and Rosberg's famous teammate Michael Schumacher this week conceded that the Brackley built car was not the quickest at the opening Valencia test.
But 24-year-old German Rosberg told Bild newspaper: "Next week at the Jerez tests the veil will continue to be lifted and a more detailed picture will emerge.
"We will certainly be at the top," the former Williams driver added.
Bild suggested that Mercedes GP may have sandbagged at Valencia by driving with high loads of fuel.
Mercedes' competition director Haug told Germany's Auto Motor und Sport: "At the first test, Ferrari did the best job of those teams in attendance.
"At the next test, and later at the first race, it may be a quite different story," he added.
Ferrari team boss Stefano Domenicali is also drawing no conclusions from Valencia.
"Maybe someone also wants to play hide and seek, and some still have to present their cars," he said on the Maranello based team's website.