After a brilliant early return from a broken leg last weekend in Germany, Valentino Rossi found he was in a little bit more pain during Friday practice at Laguna Seca.

"This track is a bit harder for me than Sachsenring and I have more pain than I had there, mostly in the ankle but also in the shoulder at the braking point before the Corkscrew," he said.

But aside from his physical worries the world champion was happy with his first day's work, which saw the Italian ranked sixth on the timesheets, and is confident that the information gathered will help him to climb the order tomorrow.

"I think we can be better tomorrow; we haven't put everything together yet to be at our full potential on the track and if we had a few more laps today we could have been a bit further up," he said.

"We used the hard tyre at the end but the track was maybe a bit dirty for it. Tomorrow we will try the soft tyre also in the front.

"I like the special livery; it's something different and fun. Tomorrow I will have a helmet to go with it!" revealed Rossi, referring to the special livery which he and Fiat Yamaha team-mate Jorge Lorenzo are using for the US GP, which features the faces of 500 fans.

Lorenzo was second fastest in practice, which saw Ducati's Casey Stoner leading the way.