Nico Rosberg set the fastest time of the opening morning of testing in Barcelona, beating his Spanish Grand Prix pole position time.
Fresh from his victory in Sunday's race, Rosberg was the busiest driver in the opening four hours of running in Barcelona, setting a best time of 1:24.374 and completing 78 laps. With Mercedes testing tyres for Pirelli on the first day, Rosberg's time was set on unmarked rubber but was 0.3s quicker than his pole lap on Saturday.
Much like this weekend's race, Ferrari was closest to Mercedes but comfortably adrift in terms of pace. Raffaele Marciello is driving the SF15-T on Tuesday and also passed the 50-lap mark as he posted a 1:26.648 to end the morning session over two seconds adrift of Rosberg.
Pastor Maldonado was third quickest for Lotus, 0.7s off Marciello. It was a more productive start to the day for the Venezeulan than Sunday's race, with 53 laps on the board and all the bodywork intact after he was forced to retire from the grand prix with rear wing damage.
Red Bull again struggled for mileage, with Daniil Kvyat fourth fastest but only registering 20 laps in the morning session. The team is likely to be careful with its running as it has suffered numerous power unit failures this season, although team principal Christian Horner says he wants to see Renault push for performance rather than reliability.
There were two debutants on Tuesday morning, with Pierre Gasly fifth in the Toro Rosso and completing 60 laps. Gasly - who finished 3.7s off Rosberg's time - will switch to the Red Bull on Wednesday.
The other new driver was Nick Yelloly who made his debut for Force India. Yelloly was originally scheduled to drive on Wednesday but an illness for Pascal Wehrlein saw the young British driver switch to Tuesday, with Esteban Ocon called up for day two as a result. Yelloly enjoyed a solid morning, completing 49 laps and ending up just 0.053s slower than sixth-placed Marcus Ericsson.
Oliver Turvey got his first run in the McLaren MP4-30 as the team worked on aerodynamic analysis. Turvey's total of 31 laps saw a best time over five seconds off the pace, and he is scheduled to have a long lunch break as the team makes substantial changes ahead of the afternoon session.
Williams' Felipe Massa was slowest in the morning, six seconds off Rosberg's pace in a session which saw no interruptions.
For live coverage of the opening day of testing, click here
Click here for Tuesday's gallery from testing at the Circuit de Catalunya