Daniel Ricciardo retained McLaren's position at the top of the timesheets in a session where a problem for Sergio Perez caused the first red flag of the season.
F1 testing continued at the Circuit de Barcelona-Catalunya on Thursday following an uninterrupted first day but Perez caused a brief pause in the running when his Red Bull stopped on track.
After watching Max Verstappen complete 147 laps on Wednesday, the Mexican was unable to return to the track before the chequered flag fell on the morning session.
The timesheets again proved positive for McLaren and Ferrari with the two teams retaining their positions at the top as Daniel Ricciardo set the pace with a time of one minute 20.355secs.
Carlos Sainz in P2 was just 0.191s back after ending Wednesday third fastest.
Pierre Gasly and Alex Albon followed with AlphaTauri and Williams beginning to show some pace.
Lewis Hamilton will once hand over to team-mate George Russell for the afternoon in a reverse of the day one order and the seven-time champion completed 40 laps across the session, although with a time that placed him ninth of 10 drivers.
In completing 71 laps, Sainz registered the most mileage of any driver with Esteban Ocon and Mick Schumacher tied second on 66 for Alpine and Haas respectively.
Valtteri Bottas completed the fewest laps for Alfa Romeo with just 21, his total following a difficult and disrupted Wednesday for the team.
F1 THURSDAY MORNING RESULTS
1. Ricciardo 1.20.355 
2. Sainz 1:20.546 
3. Gasly 1:20.764 
4. Albon 1:21.531 
5. Stroll 1:21.920 
6. Schumacher 1:21.949 
7. Bottas 1:22.288 
8. Perez 1:22.412 
9. Hamilton 1:22.562 
10. Ocon 1:23.280