Charles Leclerc topped the morning session from day seven of testing in Barcelona, heading the timesheets with a 1:16.361.
The lap time marks the fastest we've seen at pre-season testing so far this year, and is just two-tenths slower than Lewis Hamilton's pole lap time from the Spanish Grand Prix last year.
Alexander Albon was the only other driver in the 1:16's, as his time was four tenths down on Leclerc's. Lando Norris was third for McLaren, failing to break into the 1:16's by just 0.084.
Pierre Gasly followed close behind for Red Bull Racing, heading Nico Hulkenberg and Lance Stroll. Mercedes didn't do a proper short run, as Hamilton ended the session in eighth, behind the Alfa Romeo of Antonio Giovinazzi.
Hamilton set the most laps of any driver throughout the session, racking up 84 laps for the Silver Arrows. The Briton will be handing the car over to teammate Valtteri Bottas for the running in the afternoon.
George Russell was ninth, while Kevin Magnussen rounded out the timesheets in the Haas.
As well as the switch at Mercedes, Daniel Ricciardo will be climbing into the Renault in place of Nico Hulkenberg when the running resumes, and Grosjean will take over driving duties at Haas.