Click here for full morning results from day one of F1 testing
Jeson Button has topped the opening session of the first in-season F1 test at Circuit de Catalunya narrowly beating Sebastian Vettel in the Ferrari.
The McLaren-Honda driver was the only driver to use the super-soft Pirelli tyres but duly made it pay by setting the quickest time of the morning session with a 1m 23.753s – 0.01s faster than Vettel on the yellow-banded soft tyres.
After Rosberg set the early pace it was the McLaren of Button who was the first driver to set times relative to the single-lap pace seen during the Spanish Grand Prix, albeit using the red-banded super-soft tyres which weren't available for the race weekend.
In the opening hour Williams' development driver Lynn got heads turning in the paddock with an experimental double rear wing, which would be illegal under current regulations, but could be considered ahead of the 2017 rule changes.
Vettel, on qualifying runs, edged closer to Button's best time on the soft tyres with a 1m 23.763s but narrowly missed out by one-hundredth of a second against the Briton.
Rosberg followed Mercedes pattern from winter testing at Circuit de Catalunya by completing a healthy bank of laps on the medium tyres to take third on the timesheet, ahead of Lynn in the Williams and Daniel Ricciardo in the Red Bull.
Pascal Wehrlein made impressive gains with Manors updates to end the morning session sixth overall as Ocon banked in data and mileage on Renault's new engine configuration.
Grosjean had a relatively subdued session in the Haas while youngsters Pierre Gasly and Alfonso Celis Jr familiarised themselves in the Toro Rosso and Force India respectively running the team's testing programmes.