Nico Rosberg conceded that Mercedes still isn't happy with its car, despite completing the most laps today (131) and getting in several long runs.
Rosberg found better consistency with his tires on runs this afternoon, but the car still lacked the pace to match the front runners. When asked if he felt the performance of the MGP W02 is not there at the moment, he said: "Oh, that's for sure. Not yet.
"Our plan was always just to have a car to do some reliability testing. That's working out, although we still have some issues we need to sort out. We're going to look at performance soon, I think at the next test, and in the first race we'll know where we are. Until then it's difficult."
Rosberg added that he believes drivers will be able to make a bigger difference this season, due the delicacy of the Pirelli tires over long runs.
"The driver really is the key because you have to nurse the tires in the beginning, and you've got to judge it perfectly to be able to get that long run exactly the amount of laps you have to do, and save the tires until the last lap, so it's very interesting," he said.
"I'm learning a lot because, especially the tyres, it's whole new situation, first of all in terms of driving style to get the best out of them for one lap, because it's difficult. For example, the front tyires, as soon as you steer too much, they lose a lot of grip and just wash out, so you have to be very careful what you're doing with the steering wheel, how much angle you put in."