Casey Stoner played down the significance of his fastest time in Friday practice at Laguna Seca, and said he could not be satisfied until Ducati had maintained that form through the weekend.
The Australian beat championship leader Jorge Lorenzo in the opening session in America, but having not yet won a race in 2010, he was not getting carried away with his Friday performance.
"In general we have to be happy because we're setting the pace at the moment but we have to continue this tomorrow," Stoner said.
"In a lot of races this season we've been fast on a Friday but not able to improve and lost ground to the others so we want to make sure that doesn't happen here. I'm not looking to be first in practice, I would like to be first at the end of the race on Sunday."
He added that there will still plenty of areas to be improved on his Ducati.
"We tried a few different things with the setting because we still need to find a better feeling with the front and also we need to adapt the braking because there are a lot more bumps here in the braking zones than there were last year," said Stoner.
His teammate Nicky Hayden was seventh fastest, but a second off Stoner's pace.
"The session didn't go quite as well as I'd hoped but we got a good base and learned a few things," Hayden said. "Casey going so fast is a good thing for us because it shows the potential of the bike at this track even though we know he is really special around here.
"From our side, we need to work a little on the gearbox tomorrow because the engine is quite different to what we had last year. It's maybe something we need to look at tonight."