Sainz was quickest when all the cars came in at the halfway stage for a second set of tyres, and he increased his margin to nearly half a second before some of his rivals started to catch up.
Will Buller, who was also near the front throughout the session, won the battle for best of the rest to join the Spaniard on the front row, while rookie Luca Ghiotto continued his form from free practice to take third.
Fourth place went to Will Stevens, while a late showing of speed from DTM refugee Roberto Merhi landed him fifth place ahead of Marlon Stockinger, who had run as high as second to Sainz earlier on.
Any hope that the chasing pack had of deposing Sainz in the final minute was taken away when the session was red flagged with 43 seconds remaining after Zoel Amberg went off at the second chicane.
The biggest surprise of the session came further down the order, as Fortec pairing Oliver Rowland and Sergey Sirotkin both struggled throughout.
In the end Rowland could only manage 16th, with Sauber Formula 1 junior Sirotkin on the following row in 18th place.
Pos Driver Team Time Gap 1. Carlos Sainz Jr DAMS 1m33.235s 2. Will Buller Arden 1m33.359s +0.124s 3. Luca Ghiotto Draco 1m33.451s +0.216s 4. Will Stevens Strakka 1m33.549s +0.314s 5. Roberto Merhi Zeta 1m33.653s +0.418s 6. Marlon Stockinger Lotus 1m33.692s +0.457s 7. Nikolay Martsenko Comtec 1m33.708s +0.473s 8. Pierre Gasly Arden 1m33.857s +0.622s 9. Jazeman Jaafar ISR 1m33.863s +0.628s 10. Matthieu Vaxiviere Lotus 1m33.920s +0.685s 11. Roman Mavlanov Zeta 1m33.935s +0.700s 12. Marco Sorensen Tech 1 1m34.110s +0.875s 13. Matias Laine Strakka 1m34.132s +0.897s 14. Zoel Amberg AVF 1m34.178s +0.943s 15. Pietro Fantin Draco 1m34.252s +1.017s 16. Oliver Rowland Fortec 1m34.274s +1.039s 17. Norman Nato DAMS 1m34.354s +1.119s 18. Sergey Sirotkin Fortec 1m34.414s +1.179s 19. Oliver Webb Pons 1m34.460s +1.225s 20. Meindert van Buuren Pons 1m34.650s +1.415s 21. Beitske Visser AVF 1m35.383s +2.148s
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