Set-up Mercedes' only issue - Rosberg

The German was fastest in today's Barcelona test, and ended the day extremely upbeat about his car.

"I think in general there's no absolute weakness with the car at the moment," said Rosberg.

"We just need to nail the set-up a little bit better to really extract the maximum potential from the car, which I think we're not quite doing yet. We're close and maybe tomorrow we can make another step in the right direction."

Mercedes has already promised a significant aerodynamic upgrade for the season-opener, which Rosberg hopes will be the final improvement required to get the team to the front.

"We have some upgrades here, but the big package is coming for Bahrain - and it's going to be a big package," he said.

"But I'm sure everybody else has similar big steps, and then we just have to see where we are."

The German added that he would head for Bahrain feeling very confident and at home in his new team and car. He will hand the MGP W01 over to team-mate Michael Schumacher for tomorrow's final session of the winter.

"It's still early days so I'm very optimistic at the moment," Rosberg said. "That's my attitude and how I'm pushing forward, and I think today has been a good day for us.

"To repeat, it's difficult to say where you are at the moment, but I feel very confident in the car, very comfortable, which is a nice feeling for myself - I can really attack and it feels good to drive."