Lewis Hamilton was beaten to top spot by Valtteri Bottas in opening practice for Sunday's Spanish Grand Prix.
Bottas saw off his Mercedes team-mate by just four hundredths of a second at Barcelona's Circuit de Catalunya.
The all-black machines held a clear advantage over the rest of the field, with Max Verstappen the best of the rest.
The Red Bull driver, who upset the odds to win at Silverstone last weekend and now trails Hamilton by 30 points in the championship, finished third, 0.939 seconds off Bottas' pace.
Charles Leclerc was fourth ahead of Sebastian Vettel in the sister Ferrari.
Mercedes' winning run came to an end at the 70th Anniversary Grand Prix last time out after both Hamilton and Bottas struggled with their tyres.
But the world champions bounced back in ominous fashion in the first running of the weekend by dominating the time charts.
Bottas completed more laps than any other driver (33) despite a run-in with the local wildlife here. "F*** that was close, just nearly hit a bird," the Finn said over the radio. "I think I touched its wing," he added.
Four-time world champion Vettel arrived for the sixth round of the championship facing questions over his immediate future after he finished 12th at Silverstone.
The German has a new chassis for this weekend's race and he was only fractionally slower than team-mate Leclerc.
Sergio Perez is back in the cockpit after missing the past two races with coronavirus.
The 30-year-old Mexican was given the green light to compete by the FIA after returning a negative test and he finished seventh in the order, 1.6 sec slower than Bottas.
Elsewhere, Alexander Albon was eighth for Red Bull, while British driver Lando Norris finished 13th in his McLaren almost two seconds off the pace.
Williams development driver Roy Nissany was handed his grand prix debut by the British team on Friday morning.
The Tel-Aviv-born racer, 25, took George Russell's seat for the opening running but finished at the bottom of the standings, 3.9 sec down.
