Daniel Ricciardo’s leading time from the morning, which was set on the hypersoft tyre, was enough to keep the Red Bull driver on top of the charts.
The Australian clocked 165 laps in Barcelona, with the Mercedes of Lewis Hamilton ending the day second quickest ahead of team-mate Valtteri Bottas, who took over in the afternoon.
There was a changeover at Ferrari, too, with Kimi Raikkonen recovering from feeling unwell in the morning to take the wheel from team-mate Sebastian Vettel after lunch.
Between them they managed 115 laps, which was more than double that of McLaren, who suffered yet more agony in pre-season testing.
Fernando Alonso’s day started encouragingly as he moved into fourth fastest on the timesheets with 47 laps on the board. But a small oil leak forced an engine change.
McLaren did manage to get him out with just minutes to go, allowing the team to complete system checks following the change and extend their tally to 57 for the day.
Brendon Hartley banked yet more mileage for the new Toro Rosso-Honda partnership with 119 laps, with Nico Hulkenberg, Esteban Ocon and Charles Leclerc also breaking the 100-lap mark.
Leclerc's session finished a few minutes early when he slid off the track and into the gravel at Turn 12, bringing out the red flags.
Like Renault, Ferrari and Mercedes, Williams also split their running, with Lance Stroll driving in the morning and Sergey Sirotkin taking over in the afternoon. Between them, the Williams duo gathered 143 laps of data.
Running resumes on Thursday for day seven of the eight-day test in Barcelona.
You can follow minute-by-minute Live Timing from pre-season testing on the Official F1 App. For more information, click here.