Lewis Hamilton has admitted it is difficult to quantify the effect of Mercedes' latest batch of upgrades.

Hamilton was sixth quickest in both practice sessions on Friday ahead of Sunday's Spanish Grand Prix, the fifth round in the 2013 F1 World Championship, but while his initial impressions were positive, he wasn't quite so happy this afternoon in the dry, even if he was only just over three tenths off the pace.

"The team have done a great job to bring our upgrades here this weekend and whilst I could feel the impact immediately, it's difficult to quantify how much of an improvement has been made at this early stage," he confirmed.

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"After the first session this morning, I felt there was some good promise and the car was well-balanced but I was a little less happy in the different conditions this afternoon. We'll keep working on it tonight and see if we can make tomorrow a little better."

Nico Rosberg meanwhile was ninth quickest in FP1 this morning, and seventh in FP2, 0.258 seconds slower than his team-mate.

"It seems that we are quick again on one lap but we need to focus very much on achieving a good race pace," he stated. There is still a lot work to do but in general I felt that we had a good test programme today and learned quite a lot.

"It's difficult to manage the tyres in these conditions and we have seen a lot of graining today because of the lower temperatures."

"We've been working hard on improving our long runs and there are some signs of improvement today but we need to have a good look at the data this evening before making any conclusions," added team principal, Ross Brawn. "The car seems to be quite good on low fuel as we know."

"Tyre degradation is going to be a key consideration over the weekend and we must remain focused to ensure that we get the best possible performance from the car in the race. That's our priority," he emphasised.