Nico Rosberg believes Mercedes is now much closer to solving the issues that held it back early in winter testing, having put the team on top again in today's Catalunya test.
After an ominously quiet start to Mercedes' winter, Michael Schumacher jumped to the head of the order on the second day at Jerez last week, a performance that Rosberg repeated with a rapid late lap near the end of today's session.
The German warned that Mercedes still had plenty to do, but said recent gains had boosted his confidence in the W02.
"For sure we still have some problems, but it's going in the right direction," Rosberg said. "Today we learned a lot and made some progress, so things are improving."
Mercedes had reiterated during its initial struggles that it had begun testing with a car some way off its definitive 2011 specification and would only show its true potential at the season-opener, a point Rosberg underlined today.
"It was expected that we would be struggling in the winter because the car is built mainly to test reliability and it's not there for performance," he said.
"The performance is going to come in the next few weeks and then we have to wait and see where we are."
Although the first half of today's test was affected by rain, Rosberg still felt Mercedes had managed to get plenty done.
"Generally it's been good, even on the intermediates we learned a lot and then in the afternoon on the long runs we made some good progress in the end," he said.
"We definitely learned some new things again about some new aero parts and things like that, so it's been a good day and it's all going in the right direction."