Rosberg finished fastest on the opening day of the second testing session in Barcelona this week and he believes that Mercedes will be more competitive this time around.
"I am pleased, it is a good start at the moment - the car feels good and the short runs, long runs, everything is looking ok," he told Sky Sports News.
"It is definitely progress from last year, for sure we will be closer to the top guys - that is important to be heading in the right direction - so the team has done a really good job over the winter.
"It looks like all their hard work and effort has definitely been worthwhile and the car is a pleasure to drive as the balance is good."
The 2013 season gets underway with the Australian Grand Prix on March 17.