Dani Pedrosa edged out MotoGP title rival Jorge Lorenzo in the opening Friday practice session at Laguna Seca.
The championship's top two riders set a searing pace on the American circuit, both dipping into the low 1m21s - which is near outright record pace.
It seemed Lorenzo had done enough to secure top spot after two quick laps in succession allowed him to get down to a 1m21.498s, but three minutes from the end Pedrosa shaved off seven hundredths of a second to go top.
Casey Stoner also improved late on but still finished four tenths down on his Honda team-mate in third, while Ben Spies was a further 0.1s back in fourth.
Andrea Dovizioso beat Tech 3 team-mate Cal Crutchlow to fifth, although the pair were both more than 1s down on Pedrosa.
The large gaps down the field continued as Valentino Rossi sealed seventh for Ducati despite being a further half second down on Crutchlow, although LCR Honda's Stefan Bradl and team-mate Nicky Hayden - who announced a contract extension shortly before the session's start both finished within one tenth of the Italian.
The same was not true of Gresini's Alvaro Bautista, more than two seconds down on Pedrosa, or of the CRT entries.
Colin Edwards was the top CRT representative but was 3.481s down, while wildcard entrant Steve Rapp found himself more than eight seconds in arrears in the early stages.
The two satellite Ducatis also floundered. On his return from injury Karel Abraham wound up 13th, while Toni Elias standing in for Hector Barbera at the Pramac outfit was two places further back.