Nico Rosberg says there is still much work to be done on the Mercedes' set-up, branding his day as 'difficult' despite his headline grabbing single lap pace in the W06 at the Circuit de Catalunya.

Mercedes gave the clearest indication yet of its potential advantage over a single lap ahead of the 2015 F1 season as Rosberg turned in a best lap of 1min 22.792secs on the soft tyres, some seven tenths better than Felipe Massa's equivalent benchmark in the Williams yesterday.

However, though the German was pleased with that performance in isolation, he was otherwise critical of the car's handling prior to that, saying persistent set-up problems had left the car lacking in traction.

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"Today was a difficult day because even though the numbers looked like they were in the right place on the set-up, it was very far away where we needed to be, and we don't really understand that," he lamented.

"So the whole day we were chasing the set-up and trying to find the right direction and it was really difficult to understand which way to go. There was a lot of oversteer, so the rear was all over the place with poor traction, so we spent the whole day trying to move in the right direction and then in the end it was feeling more decent.

"The car was not good on the rear, it felt like the opposite of planted, a subjective opinion of course, but I always want better and better. It is a work in progress."

Indeed, Rosberg suggests the soft tyre pace - though impressive - was irrelevant as it otherwise disguises a lack of balance on the car.

"Putting the soft tyres on, that gave the grip anyway, so it doesn't matter where the balance is. I am starting to feel good in the car. My laps in the end I nailed them, so I am pleased with that."

Rosberg hands the car over to defending champion Lewis Hamilton, who will get his final pre-season run in the car tomorrow.