Valtteri Bottas led the field mid-day at the Circuit de Catalunya on the opening day of pre-season testing, the Mercedes driver leading Racing Point's Sergio Perez and Red Bull's Max Verstappen.
Bottas clocked in after a trouble-free morning with a 1m17.313 that edged Perez by just 0.062s.
Verstappen put 91 laps under his belt, the most of any driver, as he put his new RB16 through its paces.
There were surprisingly no stoppages during the session, with the teams' contenders demonstrating solid reliability from the outset.
However, Haas' Kevin Magnussen suffered a spin at Turn 9 just before the lunch break, but the Dane's car fortunately incurred no damage.
Ferrari's Charles Leclerc was unexpectedly on duty for the Scuderia, the Monegasque replacing an unwell Sebastian Vettel.
In the Williams camp, a trouble-free run for George Russell and the FW43 was a stark contrast to the belated and troubled debut endured by the team last year in pre-season testing.
The Brit, who completed a total of 73 laps, ended the morning P6 and a tenth faster than Leclerc and a couple of tenths ahead of Alfa Romeo reserve driver Robert Kubica.
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