Nico Rosberg flexed Mercedes' muscle on day one of the second Formula One test in Barcelona as he recorded Tuesday's fastest time during his race simulation.
With Mercedes giving both drivers half a day in the car it was Rosberg who had first crack at the Barcelona track, and he shone as they unwrapped their first set of soft compound tyres.
The German stopped the clock after one minute 23.022 seconds on his 66th lap - seven tenths quicker than last week's fastest time on the softs - and completed another 16 laps before handing over to team-mate Lewis Hamilton.
Double defending champion Hamilton made a stuttering start as his Mercedes stopped in the pitlane on his way out, and once he got onto the track he was unable to find a similar pace to Rosberg as he ended the day with the sixth fastest time after 89 laps.
Williams' Valtteri Bottas was Rosberg's nearest challenger as he clocked up 123 laps, the Finn deploying the ultra-soft compound to go two tenths behind Rosberg.
There were positives for McLaren as Fernando Alonso enjoyed a competitive day, while Kimi Raikkonen recovered from having his Ferrari break down at turn one to set the fourth quickest time.
New boys Haas endured a tricky afternoon as a fuel system error limited Esteban Gutierrez to only 23 laps, while rookie Rio Haryanto got some valuable track time in the Manor after an oil leak earlier in the day.