The 2008 world champion, who switched from McLaren over the winter, has admitted he is struggling with brakes this year, after Rosberg scored three poles on the trot and then won the recent Monaco grand prix.
Team boss Ross Brawn says Briton Hamilton only needs time to adjust.
But 28-year-old Hamilton said he "expected" Rosberg to be better in 2013.
"I expected him to be even stronger than he has been simply because he has been in the team for years," he is quoted by British newspapers.
Hamilton also warned that his problems with the brakes will not be quickly fixed.
"I see it being a problem for me for quite some time, until I figure it out, which I haven't done yet," he said.
And while Rosberg may have won in Monaco, Hamilton does not expect Mercedes to be challenging for the title this season.
"I don't really look at it and think we are contenders at the moment," he insisted.
Asked by Spain's Marca newspaper to name the best package of 2013, Hamilton answered: "I would say Ferrari.
"They are not yet the best in qualifying, but they have a race pace that we don't.
"We need to improve our management of the tyres, and then we'll be able to compete on par with Ferrari and Red Bull," he said.