Max Verstappen posted a dominant benchmark lap time ahead of Red Bull teammate Sergio Perez to fire warning signs for the rest of the Formula 1 weekend.
The reigning World Champion registered a 1:14.606 on the revised Catalunya circuit, a lap time three-quarters of a second faster than teammate Sergio Perez.
With the Circuit de Barcelona-Catalunya often chosen as a venue to test updates or gather learnings about a car, several teams chose to adorn their cars with aerodynamic devices and 'flow-vis' paint.
Many also took the opportunity to trial Pirelli's new prototype Hard tyres, which had been provided to teams as extra sets to use in Friday's running, which also gave FP1 a 'test session' feel to proceedings.
With teams gradually turning up their cars and committing to Soft and Medium-tyre runs, the reigning World Champion set the fastest lap of the session by far.
The Alpine drivers made the ideal start to their promising Monaco GP performance by setting the third and fifth-fastest time of FP1.
Esteban Ocon's Soft-tyre run was half-a-tenth shy of Perez's P2 time, with teammate Pierre Gasly just over one tenth of a second behind.
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The Alpine duo were split by AlphaTauri's Nyck de Vries, who made a positive start to a vital weekend for the Dutchman's career.
Aston Martin's Fernando Alonso finished in P6, with Haas' Kevin Magnussen placing in seventh in what is anticipated to be a mostly-unrepresentative session.
Ferrari's test programme involved running Carlos Sainz with the revised bodywork design, with Charles Leclerc lapping with the older-specification parts.
It was the older-specification car which finished higher in the timing screens in P7, but Ferrari appeared to be focusing on the longer runs as they evaluate the two car designs.
George Russell completed the top ten, but had early complaints of bouncing during the session, with Mercedes looking to validate their own upgrades after an unenlightening Monaco weekend. Lewis Hamilton finished the session in P12.
The only incident to speak of came at the very end of the session, when Gasly attempted to overtake Williams' Logan Sargeant into Turn 10.
The front-right of Gasly's car tagged the left-rear of the Williams, but both appeared to escape without damage.
Result Free practice 1 - Spanish
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