Defending champion Jorge Lorenzo recorded the quickest lap in free practice to set the standard at the United States MotoGP.
Lorenzo continued his fine recent form with a lap of one minute 22.056 seconds in the second session at Laguna Seca.
The Yamaha Rider's display piled the pressure on championship leader Casey Stoner ahead of Saturday's qualifying.
Stoner has seen his lead reduced to 15 points after finishing behind Lorenzo in the past two rounds.
Australian Stoner clocked the fastest time in the morning, but struggled to improve on that in the afternoon and had to be content with the third best lap of the day, 0.175secs behind Lorenzo.
Stoner's Repsol Honda team-mate Dani Pedrosa took second place.
Andrea Dovizioso was fourth quickest in 1min 22.537secs, ahead of local hopeful Ben Spies and Marco Simoncelli.
Former MotoGP champion Valentino Rossi, who has crew chief Jeremy Burgess back in the garages after a two-race absence, had to be content with 11th place just behind his Ducati team-mate Nicky Hayden.