2007 US Grand Prix winner Casey Stoner dominated the second half of Friday free practice for the 2010 event, on his way to a 0.233sec advantage over world championship leader Jorge Lorenzo.
After a slow start to the session, Stoner - without a victory since the penultimate round of last season - moved ahead of Fiat Yamaha star Lorenzo just before the midway point of the hour.
The pair were evenly matched at the front during the second half, with Lorenzo staying within 0.1sec of Stoner until the Australian suddenly found 0.434sec with two minutes to go.
Lorenzo managed to close that down to just over two tenths on his penultimate lap, while third place went to Repsol Honda's Andrea Dovizioso, who lapped just over half a second from Stoner.
Dovizioso's team-mate Dani Pedrosa, winner last weekend in Germany and last year at Laguna Seca, was one place behind the Italian, but looked visibly unhappy at being left 0.860sec from Stoner.
Pedrosa had regained fourth in the final minute after briefly losing it to top American Ben Spies.
The Monster Yamaha Tech 3 rookie finished the session ahead of recovering world champion Valentino Rossi, who began his second event back from a broken leg by being the final rider within one second of Stoner.
Former double US GP winner Nicky Hayden was seventh for Ducati, with fellow American Colin Edwards eighth on the second Tech 3 entry.
Marco Melandri was ninth for San Carlo Honda Gresini, with Pramac Ducati's Aleix Espargaro tenth and the first of those making their Laguna Seca debut this weekend.
Espargaro's team-mate Mika Kallio missed last year's race through injury and so was also completing his first laps of the Californian circuit, finishing in eleventh.
Marco Simoncelli overcame an early highside for twelfth on the second Gresini machine. Rookie Simoncelli was the only faller.
Alex de Angelis began his second event for Interwetten Honda with a hopeful 14th, having beaten Rizla Suzuki riders Loris Capirossi and Alvaro Bautista, plus LCR Honda stand-in Roger Lee Hayden.
Roger, who finished tenth in the 2007 US GP for Kawasaki, lapped 3.733sec from Stoner and 0.4sec behind Bautista.
7. Nicky Hayden
14. de Angelis
17. Roger Hayden