Mercedes was one of the form teams at the start of last year, culminating in Rosberg winning the Chinese Grand Prix in April.
But the F1 W03 fell off the pace as the year went on, and the German believes that the size of the gap to the leaders at the end of 2012 makes a strong start to this year unlikely.
"I'm confident, but to start the season as we did last year, being one of the fastest cars? Not so confident," Rosberg told reporters following the launch of the 2013 F1 W04.
"The gap was quite big at the end of last year, and first we need to close that gap.
"That's our main target for the start of the season, and I'm very confident we will manage to achieve that.
"Then the big improvement needs to be in the rate of development, because last year we really flattened off in the middle of the season."
Rosberg got a chance to try the new car on Monday morning when he performed a few laps for filming purposes ahead of the official launch, and he was happy with how it felt.
"It was a great start this morning, everything went to plan," he said. "It was all good and I could push straight away.
"The car was balanced well, so it was a very good start. As a result I look forward to driving a full test day tomorrow.
"We've made a big step over the winter. The team is working better in the factory, everything is improving."
Mercedes F1 W04 - full coverage:
Mercedes' new W04 makes debut
Mercedes W04 in pictures
Hamilton keeps expectations in check