Marc Marquez unsurprisingly set the pace in opening MotoGP practice at Austin - a track where he is unbeaten - but he was pushed hard by Yamaha's Maverick Vinales.
Honda rider Marquez was consistently the fastest rider as the track rubbered-in. He worked down to 2m06.252s four laps into his first run.
Vinales suffered a huge moment on the exit of the long Turn 18 early on his second run - just avoiding what would have been a 100mph high-side.
He recovered quickly to set the fastest time of the session at that point, a 2m06.023s. He shaved 0.002s off that on his next lap, despite having to pass a rival.
Marquez then unleashed a 2m05.311s to top the times with 10 minutes remaining, with Vinales responding with 2m05.389s - just 0.078s slower.
Reigning world champion Marquez has won every MotoGP race held at the Circuit of the Americas since it joined the calendar in 2013.
Jack Miller was third fastest on his Pramac Ducati, with a late lap of 2m05.936s just pipping the works Ducati of Andrea Dovizioso.
There was drama for Ducati at the halfway point of the session as Dovizioso stopped on track at the end of the third sector when his bike appeared to cut out while on an out-lap and then coasted to a halt. The stranded Italian had to find a lift back to the pits before hopping on his second bike.
Valentino Rossi was fifth on his Yamaha, 0.731s off the pace, ahead of an impressive Pol Espargaro on the factory KTM.
Fabio Quartararo, Cal Crutchlow and the Aprilias of Aleix Espargaro and Andrea Iannone rounded out the top 10.
Friday morning practice was the riders' first opportunity to refamiliarise themselves with the Austin track's infamous bumps, with the run between Turn 1 and 2 featuring a notable jump that destabilised the bikes considerably.
Practice one times
|3||Jack Miller||Pramac Ducati||2m05.936s||0.625s||15|
|7||Fabio Quartararo||Petronas Yamaha||2m06.332s||1.021s||16|
|8||Cal Crutchlow||LCR Honda||2m06.482s||1.171s||15|
|11||Franco Morbidelli||Petronas Yamaha||2m06.729s||1.418s||16|
|12||Karel Abraham||Avintia Ducati||2m06.732s||1.421s||14|
|17||Takaaki Nakagami||LCR Honda||2m06.978s||1.667s||16|
|19||Francesco Bagnaia||Pramac Ducati||2m07.407s||2.096s||17|
|20||Miguel Oliveira||Tech3 KTM||2m08.035s||2.724s||15|
|21||Tito Rabat||Avintia Ducati||2m08.175s||2.864s||17|
|22||Hafizh Syahrin||Tech3 KTM||2m09.338s||4.027s||15|