Ferrari's Sebastian Vettel topped the timesheet on Thursday morning in Barcelona, the German driver heading into the lunchbreak with a benchmark time of 1m16.841s.
Vettel's lap - achieved on the soft C5 tyre in the closing stages of the session - edged Racing Point's Lance Stroll by 0.277 s and the Williams of surprise top-3 runner Nicholas Latifi by 0.472s.
Barcelona's overnight showers meant that drivers had a damp track to contend with at the outset.
As the circuit progressively dried and the field switched from intermediates to slick rubber, times improved significantly.
But over the course of the morning, several drivers explored the circuit's limit, especially around Turn 5, an area which caught out Valtteri Bottas, Max Verstappen and Sebastian Vettel.
Behind the top 3, McLaren's lando Norris clocked in third, ahead of Red Bull's Verstappen. The Dutchman spun on cold tyres at the chicane early on in the session but later beached his RB16 at the Circuit de Catalunya's challenging Turn 5.
Bottas ended Mercedes' morning sixth, the Finn concluding his work session a little earlier than his rivals.
Renault's Esteban Ocon was P7 in front of Haas' Kevin Magnussen, while Giovinazzi was ninth.
The young Italian had earlier confirmed the form shown by Alfa Romeo on Wednesday by topping the timesheet, but a mistake on entering Barcelona's Turn 4 sent the Alfa bacjwards into the gravel trap and in to the wall, destroying the C39's rear wing and bringing out the first red flag of the day.
AlphaTauri's Pierre Gasly rounded off the top with 35 laps under his belt. But the fields' overall mileage was limited by the damp conditions that prevailed in the fits part of the session.
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