Sebastian Vettel says he is upbeat regarding the manner in which the Ferrari SF16-H responds to his demands, after he topped testing for a second straight day on Tuesday.
Vettel sampled Pirelli's Ultra Soft tyre to set a time of 1:22.810 during the morning session, which enabled him to finish the day seven-tenths clear of Daniel Ricciardo, who also achieved his best lap on the new compound tyre.
"So far the first thing that matters is that the car is reacting and doing what I asked for," he said.
"Now obviously it is crucial that Kimi [Raikkonen] has a similar impression. We worked on a lot of things, it's a brand new car, which means there are a lot of new bits and things are in a different place.
"We tried to understand how the car reacts, obviously we had an idea and certain expectation, but when you run the car on track it is always a bit different, you are then dealing with temperatures, cooling etc.
"Sometimes you have to stop a bit longer than necessary or than you really want, just to check, other times the driver goes off track, like I did this morning. But these things happen.
"Overall we are happy, but we know that we need now a lot of mileage, so there is a lot of work ahead of us."