Marc Marquez provisionally continued his unbroken run of MotoGP pole positions at the Circuit of the Americas, but is under investigation for baulking closest rival Maverick Vinales.
Marquez predictably led the opening runs at the Austin, Texas track with a best lap of 2m04.134s, giving himself a 0.394s buffer over nearest challenger Valentino Rossi.
But the reigning champion lost the front end of his works Honda at Turn 13 mere moments later, falling from the bike and forcing him to get a ride back to the pits to change to his second bike for the final part of the session.
On his out lap for his second run, Marquez blocked Vinales while the Yamaha rider was on a hot lap, touring on the racing line at Turn 15.
Vinales was forced to abandon his lap and gesticulated angrily at Marquez, but regrouped to grab the top spot with a 2m04.064s next time round.
Marquez well and truly obliterated that effort as he lapped in 2m03.658s, 0.406s up on Vinales - but as the session ended, it was announced the incident between the pair was under investigation.
Suzuki rider Andrea Iannone was best of the rest in third, 0.551s off the pace and one thousandth of a second ahead of Tech 3 Yamaha rider Johann Zarco.
The second works Yamaha of Valentino Rossi was fifth ahead of the lead Ducati of Jorge Lorenzo.
Points leader Cal Crutchlow (LCR Honda) was seventh ahead of Andrea Dovizioso (Ducati), an injured Dani Pedrosa (Honda) and Danilo Petrucci, one of two riders to advance from Q1.
Completing the top 12 were Alex Rins (Suzuki) and shock Q1 pacesetter Pol Espargaro.
Q1: Espargaro fastest, Miller struggles
An impressive last-gasp effort from Espargaro put the KTM rider atop the timesheets in the first qualifying segment, less than a tenth clear of Pramac Ducati man Petrucci.
LCR Honda rookie Takaaki Nakagami missed out on a Q2 spot by three tenths and will head up the fifth row of the grid ahead of Tito Rabat (Avintia Ducati) and the second KTM of Bradley Smith.
Hafizh Syahrin (Tech 3 Yamaha) led the way after the first runs in Q1, but couldn't replicate that form in the final stages of the session and slipped down to sixth and 16th place on the grid.
Argentina poleman Jack Miller struggled and could manage no better than 18th, while the Aprilias of Aleix Espargaro and Scott Redding were a subdued 19th and 22nd respectively.