Lewis Hamilton set the quickest time during the opening practice session at the Circuit de Barcelona-Catalunya as Mercedes flexed its muscles on Friday morning in Spain.
Following initial runs on the Hard tyres, which will not be favoured this weekend, the pack emerged from the pit lane on the Medium compound rubber, leading to improvements across the board.
Hamilton and team-mate Valtteri Bottas surged to the front, the Briton clocking a time of 1:21.521 to end up 0.029s quicker than the Russian Grand Prix victor.
Third-placed Kimi Räikkönen, having recovered from a moment through turn 13, was the only non-Mercedes driver to finish with a second of Hamilton, albeit 0.935s down.
Championship leader Sebastian Vettel halted on track during the closing stages of his Hard tyres run, citing a gearbox problem, though tactfully parked near the pit exit, aiding the recovery of his SF70-H.
Vettel soon emerged for his run on the Medium tyres and finished 1.079s behind Hamilton.
Red Bull occupied similar territory to the opening quartet of events, with Max Verstappen fifth, 1.1s adrift of Hamilton, and Daniel Ricciardo sixth, a further four-tenths in arrears.
Kevin Magnussen led the midfield charge for Haas, ahead of team-mate Romain Grosjean, but went off at Turn 4 after locking up with 10 minutes remaining, and parked his VF-17.
Renault’s Nico Hülkenberg and Toro Rosso’s Carlos Sainz Jr completed the top 10.
Force India claimed 11th and 12th with Sergio Pérez ahead of Esteban Ocon, with Stoffel Vandoorne 13th on another difficult day for McLaren-Honda.
Vandoorne’s team-mate, Fernando Alonso, emerged for his out lap but was pitched into a spin on the entry to Turn 3 amid an engine failure, and was forced to abandon his car.
Substantial volumes of fluid poured from the stricken MCL32 as it was craned away, with Honda later confirming an oil leak caused the problem.
Both Williams drivers ran the Medium tyres earlier than their counterparts, Felipe Massa 14th and Lance Stroll 18th, sandwiching Daniil Kvyat and Sauber pair Marcus Ericsson and Pascal Wehrlein.
Sergey Sirotkin returned to the RS.17 for his second practice run of the season but again encountered problems, completing only 10 laps due to a water leak.
