Nico Rosberg fired a warning shot to Mercedes' rivals by topping the times on the first day of the second Barcelona Formula 1 test on Tuesday.
The German driver set a best time of 1:23.022 in the morning, using soft tyres, to finish over two tenths of a second clear of the Williams of Valtteri Bottas.
Bottas was unable to match Rosberg's pace despite setting his best time on Pirelli's new ultrasoft tyres.
Once more, Mercedes' day was trouble-free, with Rosberg and teammate Lewis Hamilton completing 172 laps between them.
The Briton took over in the afternoon and posted the sixth quickest time of the day as he completed an impressive race simulation.
Fernando Alonso put McLaren in third place with a run on soft tyres, although the Spaniard was 1.7 seconds off Rosberg's pace.
Alonso triggered the first red flag of the day when he stopped on the exit of the pitlane right at the end of the morning session.
Kimi Raikkonen caused the second stoppage in his Ferrari two hours later with a gearbox problem, the Finn then spending a long time in the garage as the team worked on the problem.
Raikkonen, who covered 72 laps, finished fourth in the timesheets, ahead of the Red Bull of Daniil Kvyat, who rounded out the top five.
Max Verstappen was seventh in the upgraded Toro Rosso, ahead of the Force India of Nico Hulkenberg and Felipe Nasr in the new Sauber, which made its track debut today.
Renault wound up 10th with Kevin Magnussen at the wheel, the Dane's session coming to an early finish when he caused the final red flag of the day 10 minutes from the end when he stopped at Turn 3.
The Haas team endured its worst session of the pre-season yet, with Esteban Gutierrez managing just 23 laps in the morning before his car suffered a problem with the fuel system.
Although the American squad tried its best to get the car back out, the Mexican driver was not seen on track for the remainder of the day. He finished down in 11th position.
Rio Haryanto also had a troubled day, with an oil leak in his Manor keeping him in the garage for most of the morning.
The Indonesian was slowest of all once again after 45 laps.
|5||D. Kvyat||Red Bull||1:25.049||69|
|7||M. Verstappen||Toro Rosso||1:25.176||144|
|8||N. Hülkenberg||Force India||1:25.336||121|