Lewis Hamilton has unveiled the new Mercedes car that will carry his hopes in Formula 1 this season.
The 28-year-old, who has switched from McLaren, drove the car for a few laps at the Jerez circuit in Spain where pre-season testing begins on Tuesday.
The 2008 world champion said: "Feels like we definitely have some work to do but we were on demonstration tyres so I can't really get a feel for it.
"(It feels) different. Doesn't feel too bad at all."
Mercedes struggled last season, starting strongly and winning the third race of the season before dropping further from the pace as the year went on until by the end of the year they were struggling even to score a point.
Team principal Ross Brawn said the target was to "move forward. We have to improve from where we are. Where we are is not acceptable."
Hamilton said: "This year is about one step at a time. Finish the first race, getting on the podium, then trying to win races."
He added: "This year is about trying to get settled in the team, get a good working relationship and working to steer this car in the right direction and also next year's car.
"We have to deliver as a team, of course, but I am not a miracle worker. I'm going to work as hard as I can with the team to make steps forward.
"The target is to improve on last year. Last year they didn't have a strong year.
"They got one win, but they didn't have enough podiums, didn't have anywhere near the success they deserved."
Of his test, Rosberg said: "It all went to plan and I felt very much at home, and that's important. Tomorrow is the first day of proper testing and I'm looking forward to it very much."
Team principal Ross Brawn described the car as "a clear step forward in design and detail sophistication over its predecessor" and said he was expecting "a step change in performance compared to last season".
The new car bears a strong similarity to last year's Mercedes, which started the season in good form but dropped further and further from the pace as the year progressed.
It features a McLaren/Ferrari-style rear-end treatment aimed at using the exhaust gases for aerodynamic effect, a technology which Mercedes never got on top of last season.
The team have taken advantage of new rules that allow a "vanity panel" to cover the step on the top of the nose in the region of the front wheels in the transition between the required lower height of the nose compared to the maximum height of the top of the chassis.
Other than that, the nose profile is very similar to the 2013 car, as is the machine's general appearance.
It has retained conventional push-rod front suspension rather than, like McLaren, following Ferrari's lead in using pull-rod, which has a slight aerodynamic advantage.
The car will not have the controversial 'double DRS' system that Mercedes pioneered last year, linking the overtaking aid on the rear wing to the front for greater drag reduction, as the technology has been banned.
The team will start serious work with the new car, along with the rest of the field, on the first day of pre-season testing, which starts at Jerez in Spain on Tuesday and runs for four days.
Rosberg will drive on the first and third days; Hamilton on Wednesday and Friday.