Nico Rosberg says Mercedes hasn't revealed its full potential yet as it focuses on pushing the new W07 to the maximum to understand its breaking point at the opening pre-season test.

Lewis Hamilton handed over the new Mercedes to his team-mate Rosberg for day two of the first F1 test and he pounded around the Circuit de Catalunya for a 'marathon day' of 172 laps with the majority spent on medium compound tyres.

While Mercedes could have been expected to take the fight to Ferrari and Red Bull for top outright lap times on the softer option tyres, Rosberg says the team stuck to its robust plan to run the W07 to its breaking point in order to understand its weaknesses in a race situation.

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"It was a great day. I was excited to get in the car again and go flat out," Rosberg said. "It was a real Formula 1 marathon today, lot and lot of laps and kilometres because we need to do that to push the car to its limits and find little problems with the car.

"At some point something will break but for now nothing is broken so we need to keep on going with laps and see where the weak points are. We also did a race practice this afternoon, similar to a race but not the same, and trying to improve everything as much as possible."

Rosberg trialled a new experimental floor component on the W07, which the German driver says is 'to keep one step ahead of our rivals', and feels fourth place on the timesheet is not a true representation of his pace as Mercedes have not unleashed its full potential over a qualifying-style run.

"Lap time is not realistic where we are so far and we haven't shown our cards yet as we are still holding back," he said. "For sure the car is quick and the opposition seems to be quick. How quick we are nobody knows for now but of course we are confident."

After two days of testing Mercedes has run a combined total of 328 laps - equivalent to nearly five Spanish Grand Prix race distances - and is miles ahead of its rivals in terms of distance covered despite failing to threaten the top outright lap times so far.