The Spaniard, driving for Pons Racing, dominated both the morning and the afternoon sessions, setting the best time of the day - a 1m29.652s - before the lunch break.
Nico Muller in the Arden Caterham car and Strakka's Matias Laine completed the top three, the trio the only men to lap the Spanish track in the 1m29s during the session.
"It was almost a perfect day and I'm very pleased," said Sainz, who will be switching to DAMS for Wedneday's test.
"We worked well with the team. I've always thought that Pons Racing is a solid outfit. They're friends of mine and I really want to do a good job with them."
Russian Sergey Sirotkin was fourth fastest for ISR, while Will Buller completed the top five in the second Arden car.
Recently-crowned European Formula 3 champion Raffaele Marciello (DAMS) finished a strong sixth, while British F3 champion Jordan King posted the 15th quickest time in the morning and the 19th in the afternoon in the Carlin machine.
The Barcelona test ends on Wednesday.
Morning session Pos Driver Team Time Gap Laps 1. Carlos Sainz Pons 1m29.652s 39 2. Nico Muller Arden Caterham 1m29.704s +0.052s 37 3. Matias Laine Strakka 1m29.966s +0.314s 30 4. Sergey Sirotkin ISR 1m30.056s +0.404s 24 5. William Buller Arden Caterham 1m30.182s +0.530s 30 6. Raffaele Marciello DAMS 1m30.247s +0.595s 35 7. Daniil Move ISR 1m30.254s +0.602s 31 8. Will Stevens Strakka 1m30.329s +0.677s 34 9. Pietro Fantin Draco 1m30.465s +0.813s 46 10. Jann Mardenborough Carlin 1m30.593s +0.941s 37 11. Nick Yelloly Comtec 1m30.623s +0.971s 40 12. Marlon Stockinger DAMS 1m30.689s +1.037s 43 13. Stefano Coletti Comtec 1m30.898s +1.246s 19 14. Pierre Gasly Tech 1 1m30.912s +1.260s 33 15. Luca Ghiotto Pons 1m30.951s +1.299s 33 16. Riccardo Agostini Fortec 1m31.093s +1.441s 31 17. Oliver Rowland Fortec 1m31.313s +1.661s 29 18. Vittorio Ghirelli AV Formula 1m31.374s +1.722s 40 19. Nikolay Martsenko Zeta Corse 1m31.630s +1.978s 34 20. William Vermont Tech 1 1m31.735s +2.083s 44 21. Jordan King Carlin 1m31.842s +2.190s 30 22. Oscar Tunjo Lotus 1m31.896s +2.244s 28 23. Lucas Foresti Draco 1m31.975s +2.323s 27 24. Alice Powell Av Formula 1m32.534s +2.882s 43 25. Matthieu Vaxiviere Lotus 1m32.618s +2.966s 24 26. Roman Mavlanov Zeta Corse 1m33.731s +4.079s 36 Afternoon session Pos Driver Team Time Gap Laps 1. Carlos Sainz Pons 1m30.502s 41 2. Nico Muller Arden Caterham 1m30.606s +0.104s 37 3. Matias Laine Strakka 1m31.031s +0.529s 23 4. Pietro Fantin Draco 1m31.091s +0.589s 30 5. Sergey Sirotkin ISR 1m31.130s +0.628s 24 6. Oscar Tunjo Lotus 1m31.154s +0.652s 31 7. Will Stevens Strakka 1m31.155s +0.653s 21 8. Raffaele Marciello DAMS 1m31.185s +0.683s 28 9. Nikolay Martsenko Zeta Corse 1m31.265s +0.763s 37 10. Oliver Rowland Fortec 1m31.294s +0.792s 26 11. Stefano Coletti Comtec 1m31.302s +0.800s 25 12. Jann Mardenborough Carlin 1m31.363s +0.861s 38 13. William Buller Arden Caterham 1m31.456s +0.954s 30 14. Matthieu Vaxiviere Lotus 1m31.495s +0.993s 25 15. Jordan King Carlin 1m31.540s +1.038s 24 16. Marlon Stockinger Dams 1m31.553s +1.051s 21 17. Pierre Gasly Tech 1 1m31.601s +1.099s 40 18. Riccardo Agostini Fortec 1m31.608s +1.106s 33 19. Vittorio Ghirelli AV Formula 1m31.634s +1.132s 28 20. Nick Yelloly Comtec 1m31.979s +1.477s 33 21. Lucas Foresti Draco 1m32.068s +1.566s 32 22. William Vermont Tech 1 1m32.315s +1.813s 34 23. Daniil Move ISR 1m32.489s +1.987s 17 24. Luca Ghiotto Pons 1m32.550s +2.048s 30 25. Roman Mavlanov Zeta Corse 1m32.949s +2.447s 26 26. Alice Powell AV Formula 1m33.377s +2.875s 27
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