Marc Marquez was fastest in both the Friday practice sessions for the Grand Prix of the Americas, twice edging Jack Miller as world championship leader Fabio Quartararo condemned the Austin track as "dangerous".
On a track where he has always dominated, the Spanish Honda rider made the most of dry conditions in the afternoon to record a time of 2min 4.164sec, just 0.015sec quicker than Australian Miller on a Ducati.
Six-time MotoGP world champion Marquez posted his time close to the finish of the session.
"Cheeky fastest lap with 20 seconds of FP2 left," tweeted the Honda team.
His lap was more than 1.5 sec slower than the track record he set in 2014 but the time survived a late assault from Miller.
Marquez has taken pole position in the previous seven races at the Circuit of the Americas, which joined the MotoGP programme in 2013.
The Spaniard won the first six but fell off while leading in 2019. The race was cancelled last season when he was recuperating after smashing his right arm in crash.
He returned this season and, while he is clearly not 100 per cent, he has been much stronger at the few counter-clockwise circuits, like Austin, which put less pressure on his right arm.
Quartararo was third at 2:04.366, followed by the Hondas of Spaniard Pol Espargaro and Japanese riders Taakaki Nakagami of the satellite team.
On a circuit where the long 1.2-kilometre straight favours the raw power of the Ducatis, in-form Italian Francesco Bagnaia, who has won the last two races to jump to second in the rider standings, was sixth.
The satellite Ducatis of Jorge Martin and Johann Zarco occupied the next two places.
Quartararo was unhappy with bumps that have appeared on the track, particularly on Turns 2, 3 and 10.
"It looks like a motocross training ground," Quartararo said. "It's barely acceptable to race on...and dangerous."
Rain slowed the riders in the opening session on Friday and thunderstorms are also forecast for Saturday.
Knowing this could be the only dry practice session, riders pushed hard in the afternoon to gain a top-10 time which could mean they go directly to the second qualifying session Saturday afternoon and guarantee starting in the first four rows on the grid.
Marquez was also fastest in damp conditions in the morning session with a time of 2:15.872.
Miller made the most of the dry line that began to appear at the end of the session and was second best, 0.167sec slower than Marquez with Zarco (Pramac Racing) third.
Fastest times from the first two MotCoGP free practice sessions Friday at the Circuit of the Americas in Austin:
1. Marc Marquez (ESP/Honda) 2mib 4.164sec, 2. Jack Miller (AUS/Ducati) at 0.015sec, 3. Fabio Quartararo (FRA/Yamaha) 0.202, 4. Pol Espargaro (ESP/Honda) 0.388, 5. Takaaki Nakagami (JPN/Honda-LCR) 0.448, 6. Francesco Bagnaia (ITA/Ducati) 0.499, 7. Jorge Martin (ESP-Ducati-Pramac) 0.513, 8. Johann Zarco (FRA/Ducati-Pramac) 0.559, 9. Alex Rins (ESP/Suzuki) 0.638, 10. Enea Bastianini (ITA/Ducati-Avintia) 0.672