The Red Bull-backed Spaniard, who has taken over 2013 champion Kevin Magnussen's seat at DAMS for this year, set the fastest time of the day with just four minutes remaining in the afternoon session.
That lap came during a final flurry of activity as the session restarted for 10 minutes after Matias Laine caused a red flag by spinning.
Almost every driver in the field improved at the end, but it was Sainz who came out on top to complete a sweep of Friday's running, having also been fastest in the morning session.
In both sessions, the pacesetter's closest challengers were not necessarily those expected to be fighting for the title.
Marlon Stockinger and Zoel Amberg were second and third respectively in free practice one, while Luca Ghiotto, Jazeman Jaafar (who was an occasional frontrunner in testing), Pietro Fantin and Nikolay Martsenko led the chase in the afternoon.
Plenty of the likely frontrunners for this season were in and around the top 10 in both sessions, as Sergey Sirotkin, Will Stevens, Marco Sorensen, Oliver Rowland and Pierre Gasly all featured somewhere near the sharp end at various stages.
Only 21 cars are in action this weekend. While Carlin is not in attendance in Italy due to a lack of funded drivers, Tech 1 Racing, ISR and Comtec Racing are all only running one car each for the season opener.
Practice one:Pos Driver Team Time Gap 1. Carlos Sainz Jr DAMS 1m34.207s 2. Luca Ghiotto Draco 1m34.378s +0.171s 3. Jazeman Jaafar ISR 1m34.423s +0.216s 4. Pietro Fantin Draco 1m34.675s +0.468s 5. Nikolay Martsenko Comtec 1m34.724s +0.517s 6. Pierre Gasly Arden 1m35.058s +0.851s 7. Marco Sorensen Tech 1 1m35.084s +0.877s 8. Will Buller Arden 1m35.188s +0.981s 9. Norman Nato DAMS 1m35.226s +1.019s 10. Sergey Sirotkin Fortec 1m35.243s +1.036s 11. Will Stevens Strakka 1m35.404s +1.197s 12. Oliver Webb Pons 1m35.445s +1.238s 13. Matthieu Vaxiviere Lotus 1m35.549s +1.342s 14. Marlon Stockinger Lotus 1m35.708s +1.501s 15. Roberto Merhi Zeta 1m35.760s +1.553s 16. Matias Laine Strakka 1m35.864s +1.657s 17. Oliver Rowland Fortec 1m35.982s +1.775s 18. Meindert van Buuren Pons 1m36.004s +1.797s 19. Zoel Amberg AVF 1m36.330s +2.123s 20. Roman Mavlanov Zeta 1m36.744s +2.537s 21. Beitske Visser AVF 1m40.083s +5.876s Practice two:Pos Driver Team Time Gap 1. Carlos Sainz Jr DAMS 1m33.731s 2. Marlon Stockinger Lotus 1m33.899s +0.168s 3. Zoel Amberg AVF 1m33.986s +0.255s 4. Sergey Sirotkin Fortec 1m34.041s +0.310s 5. Will Stevens Strakka 1m34.051s +0.320s 6. Marco Sorensen Tech 1 1m34.082s +0.351s 7. Pietro Fantin Draco 1m34.143s +0.412s 8. Oliver Rowland Fortec 1m34.200s +0.469s 9. Pierre Gasly Arden 1m34.374s +0.643s 10. Norman Nato DAMS 1m34.375s +0.644s 11. Roman Mavlanov Zeta 1m34.383s +0.652s 12. Jazeman Jaafar ISR 1m34.422s +0.691s 13. Luca Ghiotto Draco 1m34.567s +0.836s 14. Will Buller Arden 1m34.650s +0.919s 15. Matias Laine Strakka 1m34.685s +0.954s 16. Nikolay Martsenko Comtec 1m34.765s +1.034s 17. Matthieu Vaxiviere Lotus 1m34.792s +1.061s 18. Oliver Webb Pons 1m34.898s +1.167s 19. Roberto Merhi Zeta 1m35.419s +1.688s 20. Beitske Visser AVF 1m35.461s +1.730s 21. Meindert van Buuren Pons 1m35.475s +1.744s
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