Valtteri Bottas admits he was surprised to see Nico Rosberg's searing single lap pace on the second day of the final F1 pre-season test in Spain, but was otherwise happy with his own performance in the Williams.

The Finn enjoyed a trouble-free day in the Williams FW37, his time of 1min 23.995secs - achieved on the soft tyre - going second fastest on the day, albeit four tenths down on team-mate Felipe Massa's benchmark from the day before.

However, it was Rosberg's 1min 22.792secs lap on the same rubber that caught Bottas' attention, admitting the pace of the Mercedes has taken him by surprise.

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"It was a good day for us, a completely trouble-free day, which is always nice. More performance work today and to try and squeeze a bit more out of the lap time.

"It felt OK, but a little bit surprised by the speed of the Mercedes. It looks like it [they have an advantage], but it is difficult to say how much further they are in front.

With one day left in the car, Bottas says he feels ready for the Australian Grand Prix, which gets underway in less than two weeks' time.

"We are really looking forward to the opening round. We know we can be competitive, but we still have work to do. There is two days left of testing and we are going to have a busy plan. We are ready to race, but there are things we can still do better and we can work it out in the next two days."