The Brackley-based team was forced to abandon its efforts for the day after just 14 laps on Tuesday, because modifications to its wiring loom are needed overnight after an electrical problem stopped the car out on track.
But although the reliability issue was a setback for Mercedes, Rosberg says early impressions of the car are encouraging.
"Very good, very positive," he explained. "I can't say if it was quick or not, but feeling wise I was comfortable. I could push, the balance was OK and it was very enjoyable to drive."
Despite feeling upbeat about the car, however, Rosberg was equally well aware that Mercedes' task was not just to improve, but to leapfrog other teams.
"I can guarantee you it [the car] will be a big step forward, but everybody else is also moving forward," he said. "We are hoping that we have closed the gap.
"Last year [the gap] was nearly more than one second at the end of the season, and we have to get closer for sure. And we will, we will get closer.
"I think they have given us a good base and now it is up to us to make the most of it now, find out the problems and also improve it.
"We also need to give feedback to the factory - what are the main things holding us back? What are the main things that we need to keep on? That is what it is about, but it is going in the right direction."