Reigning MotoGP champion Marc Marquez set the pace in first practice at Austin, beating nearest challenger Valentino Rossi by a little under four tenths of a second.
Marquez sat atop the timesheets virtually throughout the 45-minute session at the Circuit of the Americas, a track where the Spaniard is unbeaten, ending up setting a best time of 2m05.530s.
The Honda rider was more than a second clear of the field in the early stages, which were characterised by a dirty track surface, but Marquez's rivals were able to close in on their final runs.
The track has undergone a diamond-grinding process in recent weeks, in an effort to eliminate the track's notorious bumps, but this has left a lot of dusty residue on the surface.
Maverick Vinales was the first man to get within half a second of Marquez, but it was his Yamaha teammate Rossi who beat him to second spot, 0.396s off the pace with Vinales 0.727s down in the end.
Ducati's Jorge Lorenzo was the final rider to get within a second of Marquez in fourth place, ahead of LCR Honda's points leader Cal Crutchlow and the second works Desmosedici of Andrea Dovizioso.
Rookie Takaaki Nakagami (LCR) was a surprising seventh, ahead of lead Suzuki rider Andrea Iannone and Johann Zarco (Tech 3 Yamaha), who lost time early on when his bike stopped out on track in the opening minutes and then had a lap deleted for exceeding track limits.
Alex Rins (Suzuki) completed the top 10 ahead of Aprilia duo Aleix Espargaro and Scott Redding, who ran second in the early stages.
An injured Dani Pedrosa struggled to 21st place having been declared fit to ride at COTA on Thursday, 3.581s off the pace of teammate Marquez at the head of the field.