Nico Rosberg says Mercedes still has work to do to improve its performance in high-speed corners following the conclusion of the three-day F1 test at Mugello.

Rosberg spent the final day of running in Italy focusing on tyre evaluation as Mercedes looks for ways to get more performance from the Pirelli rubber.

Speaking after the test, Rosberg insisted that the time spent at Mugello had shown Mercedes there is still work to be done in high-speed corners with it proving to be a weak point thus far for the W03.

"It was indeed a good day and we were able to cover a lot of the programme that we set ourselves," he told the official F1 website. "What was really important was that we now understand why Shanghai was going so well and Bahrain a lot less well. And, of course, we have also been focusing on the next race, the Spanish Grand Prix.

"I think it is key to a good season this year to understand exactly why you have problems with the tyres at one circuit and then do extremely well on another. And I can say that we've learned a lot that will help us at the coming races. I have to say that this track [Mugello] is not exactly to our liking because it consists of almost only fast corners - and that is where we are not that strong at the moment. It's an area where we definitely have to put some thought in to understand why it is that way, as it was definitely the most crucial reason we lost positions in Bahrain.

"Leaving that aside, I can say that we've been able to cover a lot of mileage and we have learned a lot, especially on the set-up side. That's all I can say."

While some have questioned the value of a test at Mugello, Rosberg said there were plenty of reasons why the session was beneficial.

"I am not into the budget reasoning," he said. "I only can answer from a driver's point of view and I have to say that it made a lot of sense for me to do this test, as I can see that it helps the whole team and me."