Mar.5 (GMM) Toto Wolff, Mercedes' new shareholder and director, has played down claims the Silver Arrows is a definite title contender for 2013.
"Mercedes looks very strong," Ferrari technical director Pat Fry is quoted by Speed Week.
But Austrian Wolff said of the raised expectations: "I have to dampen them because they are quite simply not realistic."
He said winter testing is not a good form guide, particularly if some teams are deliberately hiding their performance.
"Red Bull for example showed nothing," said Wolff. "We won't know until Melbourne what's really going on," he told the German news agency DPA.
Team newcomer Hamilton, meanwhile, also seemed initially excited by the late-Barcelona test pace, admitting he now expects to win a race.
But Spain's El Confidential quotes him as saying: "There's no reason to be impressed. People should not get excited.
"It's important for everyone to have their feet on the ground."
Teammate Rosberg, in contrast, has sounded upbeat throughout the winter, openly admitting the W04 is much better than its 2012 predecessor.
But the real question is how it will stack up against the competition.
"Amongst others, the big unknown is Red Bull," the German is quoted as saying.