Sebastian Vettel says F1 upgrades are about quality, not quantity, after seeing Mercedes' heavily-revised W08 challenger dominate Friday practice ahead of the Spanish Grand Prix.

Barcelona's race is traditionally when the development battle steps up a level, with the race held the doorstep of every team's base. Mercedes rolled out on Friday with a radical slim-line nose on its upgraded car, while Red Bull appeared to have cut the gap to Ferrari with its own B-spec car in FP2.

Vettel himself had a messy day, one which included some gearbox trouble in FP1 and a run across the gravel in FP2, and he believes there is plenty to come on Saturday before qualifying.

"So far pretty bad," he said, summing up his day. "I struggled a bit with the car balance in general so conditions today seemed to be quite challenging. I think we would have loved to do a bit more running, get a bit of a better feel for the car. It was OK but as I said, we can improve."

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Despite seeing Mercedes finish one-two in both free practice sessions, Vettel says it is too early to judge whose upgrade package has been more successful.

"I feel good. Obviously today I don't feel so good inside the car, it was quite slippery but I think it was due to the conditions, I think everybody was struggling a bit. I don't know who brought the most updates, I don't think it matters who brought the most, I think it matters who brought the best. Some of them are more visual than others. I'm happy with what we brought, some of the stuff you can see, some of the stuff you will not see, so no secrets but the usual work."

When asked specifically about Mercedes' early pace, he said: "I don't know what Mercedes was up to. It looks like it will be between them and us, but as I said I wasn't happy. I think there was quite a lot left on the track which I didn't get to, didn't find yet. So need to have a look tonight."