Sauber rookie Felipe Nasr topped then timesheets on day three of the Jerez F1 test but admitted that he didn't expect to replicate the achievement too often in 2015.

The GP2 Series graduate took over on top after Sebastian Vettel handed the previously pace-setting Ferrari to team-mate Kimi Raikkonen, and completed 108 laps of Jerez en route to setting a target of 1m 21.545s. Raikkonen ended the day in second spot, with Mercedes' Nico Rosberg third fastest, but Nasr acknowledged that keeping the 'big guns' at bay may prove to be harder as the season goes on.

"I think we could still get some margin on the car to improve [the time] but, in general, it was good," he said, "It felt good to go to the medium and soft tyres and I think we got a good understanding of both [especially] for myself as this is all new for me. I never tried the soft tyres with Williams last year because, in testing, it was never the aim to try them, so I'm just getting used to everything and, so far, it seems a good beginning for the team and myself. I wouldn't say it was maximum performance but they were decent times on the soft tyres.

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"To be honest, I don't think we're going to be leading sessions in other situations. I think the Mercedes-engined cars are hiding a little bit - I didn't see them running like they could do, at 100 per cent, but you never know. It's testing and we don't know the programmes the others are doing. I'm just happy we did ours and I'm looking forward to Barcelona now."

Unable to directly compare last year's Williams, which he drove in testing and Friday free practice, Nasr was nevertheless content that the Sauber was a step forward on what the team had last season.

"There's always something to improve, some areas you try to push on and, as I said earlier, there's still some margin to go," he reflected, "We could improve on [the rear end downforce], but it's clear that Ferrari has made a step on last year and I think there's a bit on our chassis side also. [Stability under braking] feels okay at the moment - it didn't seem to react anything strange - [but] the balance is on the understeer side and I don't like that too much!"

Nasr's day was marred by a minor spin in the damp early going, but the Brazilian insists that that was merely the result of him experimenting in order to explore the limits of the car and its Pirelli tyres.

"It was on the quick left-hander and, on the exit, there was a wet patch and, as soon as I touched it, then I just went off. No drama..." he explained, "I think I was the only one to drive on dry tyres at that time, purely for myself to know how much you had to go to cross over to the slick tyre. It is something I have to try now - I don't want to try it in a race - so it was the right time to do it and these things can happen.

"It is good that I can find the limit, more or less what the tyres can cope with, considering the track, and it was nothing major, completely normal. It is better to have an understanding of the situation than not to and be surprised in a race weekend.

"I think, every time I am in the car, there is something that I am learning so I'm progressing and have a better understanding. Coming into a new team and a new car, you don't know what to expect, so you have to start from the very beginning and take it from there. These two days, I got to learn a lot of things quite quickly and I'm sure there is more I can [learn]."