Marc Marquez led the opening MotoGP Americas Grand Prix practice by 0.078 seconds as Andrea Dovizioso suffered a mechanical issue.
Marquez has won at the Circuit of the Americas every year since it joined the calendar in 2013, and set the early pace aboard his Honda, resisting a challenge from Yamaha's Maverick Vinales to remain fastest at the chequered flag.
The world championship leader set the benchmark at 2:06.252s in the opening stages to go 1.3s clear of Yamaha's Valentino Rossi.
Rossi remained Marquez's nearest challenger in the early stages, with the Italian managing to cut his advantage on the timesheets down to nine tenths with 15 minutes of the session gone.
Cal Crutchlow, on the LCR-run RC213V, moved ahead of Rossi four minutes later, and got to with 0.542s of Marquez before reducing his deficit to just two tenths.
Marquez's run was interrupted at mid distance, as Vinales guided his M1 to a 2:06.023s, which he improved on his follow-up tour by 0.002s.
With just over 10 minutes left in the first session, Marquez fired in a 2:05.311s to establish a 0.7s advantage over Vinales, with Yamaha team-mate Rossi in third spot the only other rider to be within a second of the Honda man.
Marquez's time remained in tact through to the chequered flag, though Vinales mounted pressure late on with a 2:05.389s to keep the Honda rider within a tenth.
Dovizioso's session was halted briefly by a mechanical issue on his Ducati as he exited Turn 16 halfway into FP1.
The Italian was forced to abandon his Desmosedici trackside, but was soon out again on his second GP19 and ended the morning fourth fastest, with Pramac's Jack Miller just three hundredths ahead.
Rossi was shuffled back to fifth in the end, with Pol Espargaro comfortably top KTM runner in sixth ahead of Fabio Quartararo on the Sepang Racing Yamaha and Crutchlow.
Aleix Espargaro headed Aprilia team-mate Andrea Iannone to complete the top 10, with works Ducati man Danilo Petrucci almost a second back of his team-mate in 13th.
The Suzuki duo of Joan Mir and Alex Rins completed the top 15 ahead of Johann Zarco [KTM], while the still-recovering Jorge Lorenzo was a low-key 18th on the sister Repsol Honda.
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