Marc Marquez headed Jack Miller by 0.353 seconds in first practice for the MotoGP Argentine Grand Prix, while Maverick Vinales was a lowly 18th.
Marquez took over top spot early on in the first 45-minute session of the weekend, and remained in charge through to the chequered flag, ending the morning as the only rider in the sub 1:40s barrier with a 1:39.827s.
Johann Zarco led a KTM one-two in the opening moments of FP1, the works man leading Tech3's Miguel Oliveira by four tenths of a second with a 1:42.237s.
Marquez soon deposed last year's race runner-up with a 1:41.129s, with the Honda rider elevating himself seven tenths clear of the pack on his follow-up effort of 1:40.673s.
Before long, Marquez – unusually running the soft front tyre – had a second in hand over the rest, with Aprilia's Aleix Espargaro his nearest challenger after 15 minutes of running.
Marquez's advantage was cut down to just under six tenths by Petronas Sepang Racing Team rider Franco Morbidelli midway through the session, before Qatar race winner Andrea Dovizioso leaped up to second, 0.457s adrift.
Another brace of session-best laps were fired in by Marquez with just over 15 minutes remaining, the best of which a 1:40.042s which once more put him seven tenths clear of the field.
A late flurry of laps led to Pramac's Jack Miller getting to within just over a tenth of the Honda man, though Marquez re-established his authority with a final gambit of 1:39.827s.
Miller held onto second spot ahead of 2018 Argentina winner Cal Crutchlow on the LCR Honda, while Morbidelli led Aprilia's Andrea Iannone.
Zarco remained in the top six on his KTM, with Dovizioso and Fabio Quartararo [SRT] shadowing. The top 10 was rounded out by Suzuki's Alex Rins and the works Yamaha of Valentino Rossi.
Rossi's team-mate Vinales had a low-key session on the sister Monster-backed factory M1, with the 2017 Argentina winner over a second off the pace in 18th.
Jorge Lorenzo similarly flew under the radar on his Honda, as the triple MotoGP world champion continues to recover from his pre-season wrist injury and a fractured rib he picked up in Qatar.
He ended FP1 in 14th behind the Espargaro brothers, with Pol [KTM] heading Aleix. Crashes for Ducati's Danilo Petrucci and Suzuki's Joan Mir at Turn 1 halfway into the session left them 20th and 19th on the timesheets.
