Valtteri Bottas put himself at the top of the timesheet in Barcelona Wednesday morning, the Mercedes driver edging out Ferrari's Antonio Giovinazzi by 0.068s before the day's lunch break.
Both drivers clocked in with 60 laps on the board, exchanging fastest times in the latter part of the morning session, with Bottas having the final word with a run in 1m16.904s - a new Barcelona lap record - using Pirelli's supersoft tyre while Giovinazzi's time was achieved with the hypersoft compound.
The young Italian was actually conducting his second day of testing having run on Tuesday with Sauber.
Kevin Magnussen, also on the hypersoft rubber, was 1.307s adrift from Bottas in third position, while Lando Norris concluded the morning fourth for McLaren, posting his time in the final hour on medium tyres.
F1 rookie Jake Aitken enjoyed a positive baptism of fire with Renault, setting the fifth fastest time over the course of his 63-lap workload.
Jake Dennis, the other fresh face facing an introduction to F1 with Red Bull focused on putting the team's RB14 through its paces on medium tyres. The Brit totaled 38 lasp, the fastest of which left hil P10 in the morning standings.
Stoffel Vandoorne picked up where Norris left off yesterday in the McLaren Pirelli car while Nicholas Latifi did the same for Force India as the two drivers sampled the F1 supplier's 2019 compounds.