The first day of the Australian Grand Prix was marred for the team when technical problems resulted in a premature end to the second practice session for both drivers but third place on the timesheets for Rosberg offered much promise for the season.
"That was an interesting day for us," Rosberg told reporters. "There are a lot of new things to learn with the car and we made some good progress over the day.
"We can certainly be happy with our testing programme when you compare where we came from last year."
The great young hope of German racing until he was eclipsed by three-times world champion Sebastian Vettel, Rosberg finally got his breakthrough race win in Shanghai last year - his seventh season in Formula One.
Having outperformed his country's greatest driver, Michael Schumacher, in the Mercedes team over the last three years, the 27-year-old now faces perhaps an even tougher challenge in the shape of 2008 world champion Hamilton.
The two know each other well from their days as team mates in karting back in the early 2000s and Rosberg visibly bristled earlier this week when asked if the Briton would be playing a mentoring role at Mercedes.
"Mentor, definitely not," he told reporters. "Of course we get on well and he's one of the best out there, and for sure, there's always something my team mate does better than me.
"I'm trying to understand that and become a better driver myself. I did that with Michael also, it was a very interesting experience to race with him.
"I know a lot about his strengths from karting so not many things are new. It's completely different from Michael, it's good."
On Friday, Rosberg was just behind the Red Bulls of Vettel and Mark Webber on the timesheets when his session was ended by a faulty gearbox, while Hamilton lapped seventh quickest but ended his day in the trackside gravel after a bodywork problem.
"It's always important to beat your team mate," Rosberg said.
"There's always two factors out there, outright result and beating your team mate. My team mate is one of the best, it's a huge challenge and that's what I'm here for, that kind of challenge."
Rosberg's experience at Albert Park last year has made him cautious about taking too much encouragement from pre-race pace.
Strong both in testing and then the practice sessions, Mercedes finished the opening weekend of the 2012 season without a single point after Schumacher retired and Rosberg finished 12th.
"Testing quick laps? That's completely irrelevant because last year we were also quickest in testing but were nowhere in the first race," he said.
"But I'm confident that we're in a better position and growing as a team, going in the right direction."
Rosberg said the two factors that had hurt Mercedes last year were rear tyre degradation and the pace of development during the season.
"I know we have resolved them," he said. "At the moment, the development rate is really good but we'll see if we can maintain that.
"Because it's from now on that it really counts. At the moment, we are closest to the fastest, so hopefully we can close that gap up."
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