The Red Bull-backed DAMS driver fended off an attack from a fast-starting Nikolay Martsenko into the first corner, and from then on set about building a massive lead.
Martsenko had a lonely race to second, comfortably leaving behind the scrap for third place.
Zoel Amberg held onto the final podium slot in the early stages, having taken advantage of Oliver Rowland and Roberto Merhi running wide at the first corner at the start.
Rowland appeared to have no answer for Amberg's speed down the backstraight, and he eventually got ahead after a drag race that was decided in the braking zone for the tight hairpin towards the end of lap 13.
Rowland ran Amberg wide, which allowed Marlon Stockinger through to fourth. The Lotus junior initially fell back from Rowland, but by the end of the race he was on the Briton's tail and forcing him to defend his third place.
Amberg came home fifth ahead of a fierce fight between Merhi and Matthieu Vaxiviere, who stormed through the field from 13th on the grid.
The Frenchman got alongside the ex-DTM racer in the closing stages, but he was edged off the track, and never quite mounted another challenge again after that.
As for Sainz's former co-championship leaders, fellow Red Bull junior Pierre Gasly came home ninth after going off at Turn 1 and picking up some nose damage during the race, while Will Stevens was unable to build on a good first lap, finishing 12th from 15th on the grid.
Results - 25 laps
Pos Driver Team Time/Gap
1. Carlos Sainz Jr. DAMS 43m14.984s
2. Nikolay Martsenko Comtec +14.335s
3. Oliver Rowland Fortec +24.291s
4. Marlon Stockinger Lotus +24.488s
5. Zoel Amberg AVF +33.825s
6. Roberto Merhi Zeta Corse +34.652s
7. Matthieu Vaxiviere Lotus +34.942s
8. Sergey Sirotkin Fortec +37.387s
9. Pierre Gasly Arden +38.622s
10. Pietro Fantin Draco +42.885s
11. Norman Nato DAMS +43.309s
12. Will Stevens Strakka +46.232s
13. Jazeman Jaafar ISR +48.853s
14. Meindert van Buuren Pons +49.922s
15. Marco Sorensen Tech 1 +51.361s
16. Luca Ghiotto Draco +54.250s
17. Roman Mavlanov Zeta Corse +57.127s
18. Beitske Visser AVF +59.760s
19. Matias Laine Strakka +1m05.335s
Oliver Webb Pons 1 lap
William Buller Arden 1 lap
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