Fernando Alonso said he wasn't shocked to see Red Bull lead the way in Friday practice for the United States Grand Prix.

Points leader Sebastian Vettel and his Red Bull team-mate Mark Webber led the way during the Friday sessions with Alonso best of the rest in third as he seeks to keep his championship hopes alive.

The Spaniard admitted it wasn't a surprise to see defending champion Vettel at the head of the pack as F1 sampled the Circuit of the Americas for the first time, but said he wasn't too downbeat with his own performance on track.

"It's no surprise to see the two Red Bulls ahead of the field: in the last few races they have had the best package, therefore today's result is logical," the Ferrari ace said. "We know what are our strengths - team work and reliability - and we will make the most of them in the fight. The updates we have brought here seem to work, but we still need to look more closely at the data before having a definite answer. I am happy about this, but it's also true that while we are making small steps forward with each passing race, so too are the others, so the distance between us remains unchanged.

"Would I be happy to be third again tomorrow afternoon? Sure, but this is only Friday and we have been in this position before and then on Saturday other cars have got ahead of us. The circuit? A nice surprise, even if we had an idea of what to expect from the simulator: it's very demanding, especially the first sector which is very fast and where you can really feel the acceleration of a Formula 1 car. The track is very smooth and not at all porous: it reminds me a lot of Portimao in Portugal, so if it was to rain, it could be like driving on glass."

Team-mate Felipe Massa meanwhile was sixth fastest and, like a number of his rivals, the Brazilian admitted that tyres could be key to the remainder of the weekend.

"Today was given over above all to getting to know this new track: we did a lot of laps, mainly with this in mind," he said. "In the morning, the track was dirty and very slippery: I can't remember ever driving on a surface that offered so little grip! Then the situation improved and we began to get a bit more confidence and find the right lines. I like the track a lot: it's fun driving here and I think we will see a great show over the weekend.

"The tyres? Definitely hard! It will be important to work out how to get them up to temperature, especially as far as qualifying is concerned."