Valtteri Bottas continued his recent good form by completing a practice double for the Spanish Grand Prix.
Bottas topped the time charts in Friday's opening running at Barcelona's Circuit de Catalunya before also going quickest in the day's final session.
Lewis Hamilton finished a distant fourth this morning, but improved to second, just 0.049 seconds slower than his Mercedes team-mate.
The Silver Arrows have recorded a perfect start to the new campaign and appear to hold the upper hand on Formula One's first stop-off in Europe.
Charles Leclerc finished third for Ferrari, one place ahead of team-mate Sebastian Vettel, but both drivers were a third-of-a-second slower than Bottas.
Vettel is already 35 points off the championship summit, with Leclerc five points further back. And, despite boasting an updated engine here, they must find more speed overnight to end Mercedes' dominance and claim their first win of the year.
Remarkably, it is approaching nine months since Vettel won a race - the Belgian Grand Prix last August.
Earlier, Lance Stroll crashed out of opening practice after he lost control of his Racing Point car at Turn 9.
The 20-year-old Canadian ran off the road on the entry to the corner before colliding head-on with the barriers.
Stroll walked away unscathed from the incident, but sustained extensive damage to the front of his car.
The Racing Point mechanics performed a stellar job to ensure his car was ready for the afternoon session - Stroll finishing 13th.
Red Bull driver Max Verstappen ended the day in fifth, 0.751 sec off the pace. All of the teams have brought upgrades to this race, and Haas will be pleased with their revisions.
Romain Grosjean finished sixth ahead of Red Bull's Pierre Gasly. British teenager Lando Norris finished 16th, 1.75 sec slower than the front-runners. His McLaren team-mate Carlos Sainz was ninth.
