Nico Rosberg says Mercedes still needs time to see the real potential of its car following two troubled days of testing at Valencia.
The German completed little mileage yesterday due to a hydraulic issue, and was hindered by other problems during today's session.
He still managed 69 laps on his way to the seventh fastest time, but he reckons there are still problems to solve before the team can focus on the car's speed.
But Rosberg insisted he was very happy with the handling of the car right from the start.
"The feeling remains that I feel very comfortable in the car, which is great - straight away," Rosberg said. "And that is a nice thing to have. But then today to really push there were a few things that we need to sort out.
"All of the main systems like KERS and the main wing often were not working or we did not have them on, and that then has a big impact on lap time - and that is lap time that we will find in the next few days or weeks.
"And other than that, there were a few things like with braking with KERS that you need to learn. You need to be confident on the brakes and with the KERS coming in. It was often a bit difficult with the confidence of the braking, and it is just a matter of getting used to it."
He also admitted handling all the new systems was a real challenge.
"I learned how to handle the KERS for example with all the buttons and systems, it is very complicated for sure and it is going to take some time to get used to it.
"So that was quite a challenge today, also because we had some issues with it, so even out on track it was doing some strange things sometimes. It will be easy to sort them out for sure but today it wasn't working 100 percent."