Nico Rosberg was left concerned by "a difficult day" for Mercedes at the start of the final test.

Having emerged as the early favourites following impressive speed and reliability during the opening two pre-season tests, Mercedes was hampered by what Rosberg described as "a few small problems" which limited him to 89 laps. Despite still completing more than a race distance, Rosberg said the issues the team had hit during Thursday's running was proof that it isn't having things all its own way.

"That was not a fantastic day for us," Rosberg said. "We tried some new parts on the car and you find out a lot through doing that. But unfortunately we did not only discover good things today. I managed to do a couple of good laps before lunch but in the afternoon we had a few small problems which ended my running a bit earlier than planned.

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"This shows that we are pushing everything to the limit and we still have a massive challenge in front of us during the last three days of testing. We need to maximise our time here to be prepared for Melbourne. But after a difficult day overall we are on the right path."

Mercedes' executive director (technical) Paddy Lowe explained the plans behind the team's schedule in Bahrain and insisted the problems which occurred had been clearly identified already.

"We've re-grouped after last week's test and come back with the intention of running as close as possible to our Melbourne race specification with the F1 W05," he said. "In addition to completing our final preparations from the reliability and performance perspectives, we have also brought a number of aerodynamic updates to the car for evaluation throughout the week. We've also been looking at a number of problems which arose during running last week and have introduced solutions for them.

"This first day got off to a slightly hesitant start, with a few minor issues on our side combined with a number of red flags interrupting proceedings. Once we got going, however, Nico got straight into some relatively long runs on the medium tyre; commencing each with race start procedures, ending with practice pit stops and evaluating a few setup variations in between.

"By lunchtime and into the early part of afternoon, however, a number of problems with the car began to emerge which hindered Nico's progress and eventually brought our running to a halt an hour ahead of schedule. We understand what went wrong and will be reviewing it overnight with the aim of having a better day tomorrow."