The German edged out Kimi Raikkonen's Lotus by 0.007 seconds to lead the way as the second winter test commenced.
"The car feels decent out there. That's a good start," Rosberg said.
"It's a much, much better base. You can feel its centre of gravity is lower. The aero is better. The platform is better.
"It feels a lot stronger. It's a pleasure to drive because it's a lot faster than last year."
Rosberg's only note of caution was over reliability.
Gearbox issues meant he only completed 54 laps on Tuesday compared to the benchmark 110 of Ferrari and Fernando Alonso.
Mercedes had also suffered from technical issues in the opening test at Jerez.
"I didn't get many laps in," Rosberg said.
"That's a bit of a pity, for sure. We have to work on that.
"We have a few too many reliability issues at the moment. Other than that, it's definitely going in the right direction."
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