A scary high-speed crash for reigning MotoGP champion Marc Marquez punctuated the opening session of the premier-class weekend at the Grand Prix of the Americas.

Marquez, who counts COTA among his very strongest circuits, crashed at the 10-minute mark of the session as he tipped down into the fast downhill Turn 10 right-hander, losing the front rapidly at 190km/h.

The front just went for @marcmarquez93 and that was a FAST one 👀#USGP 🇺🇸 pic.twitter.com/i0EtTlOKuW

He slid into the gravel at speed, bouncing through the gravel trap into the airfence, before being helped up and getting a lift back to the Ducati pit garage on a scooter - where he was attended to by medical officials.

"Marc is fine but he has a huge bruise [abrasion] on the right arm and on the left hand," said Ducati team manager Davide Tardozzi.

"[MotoGP medical officer] Dr. [Angel] Charte visited him, he said [it's] nothing important - it's painful but it's not concerning the bones, so that means if he wants he can ride again.

"But for sure it's painful. No x-rays, it's just a problem of skin."

And Marquez did indeed duly return to the track in the final minutes, lifting himself up to fourth place in the classification - despite being the only rider in the field limited to single-digit laps.

KTM rider Pedro Acosta had led in the early going, and used fresh rubber late on to reclaim the top spot - by a tenth over VR46 Ducati's Fabio Di Giannantonio.

Jorge Martin was the fastest of the Aprilia riders and also the best of those to have stuck with the same tyres through the session, his best effort just 0.221s down on Di Giannantonio.

FP1 results

1 Pedro Acosta (KTM) 2m01.710s
2 Fabio Di Giannantonio (VR46 Ducati) +0.103s
3 Jorge Martin (Aprilia) +0.221s
4 Marc Marquez (Ducati) +0.378s
5 Raul Fernandez (Trackhouse Aprilia) +0.495s
6 Alex Marquez (Gresini Ducati) +0.589s
7 Luca Marini (Honda) +0.650s
8 Ai Ogura (Trackhouse Aprilia) +0.673s
9 Marco Bezzecchi (Aprilia) +0.710s
10 Franco Morbidelli (VR46 Ducati) +0.755s
11 Pecco Bagnaia (Ducati) +0.796s
12 Jack Miller (Pramac Yamaha) +0.916s
13 Maverick Vinales (Tech3 KTM) +0.917s
14 Enea Bastianini (Tech3 KTM) +1.003s
15 Alex Rins (Yamaha) +1.035s
16 Brad Binder (KTM) +1.051s
17 Joan Mir (Honda) +1.162s
18 Fabio Quartararo (Yamaha) +1.232s
19 Toprak Razgatlioglu (Pramac Yamaha) +1.322s
20 Johann Zarco (LCR Honda) +1.341s
21 Diogo Moreira (LCR Honda) +1.410s
22 Fermin Aldeguer (Gresini Ducati) +1.609s