Dani Pedrosa led the field at the end of MotoGP FP1 around the Termas de Rio Hondo circuit in Argentina with a 1m 40.303s lap.
Cal Crutchlow made it a Honda 1-2 less than half a tenth behind.
Andrea Iannone continued Suzuki’s strong start to 2018 by finishing the session in third position, less than a tenth behind Pedrosa’s benchmark.
The top five was rounded out by a pair of satellite riders, with Qatar poleman Johann Zarco putting his Tech 3 Yamaha fourth ahead of Jack Millers Pramac Ducati.
Pedrosa’s team-mate and reigning world champion Marc Marquez ended FP1 in sixth place ahead of the first of the Movistar Yamaha’s of Valentino Rossi.
The two factory Ducati GP-18’s had a subdued opening practice session, Andrea Dovizioso leading Jorge Lorenzo in eighth and ninth respectively.
Danilo Petrucci completed the top ten on the second of the Pramac machines, just under eight tenths away from the fastest time.
Arguably the star of the session, however, was Hafizh Syahrin, the Malaysian rookie managing to put his Tech 3 Yamaha 11th less than eight-tenths from the top spot, and just half a second from team-mate Zarco.
Maverick Vinales meanwhile continued his 2018 struggle by ending the session all the way down in 13th place, just ahead of the first of the KTM’s piloted by Pol Esparagro.
The fastest of the Aprilia’s was Aleix Espargaro, who managed to slot his RS-GP just ahead of Vinales in 12th place, while Alex Rins had a tough opening session on the second of the Suzuki’s in 21st position.