Sergey Sirotkin went fastest for Fortec on the third and final day of Formula Renault 3.5 testing at Aragon.
The Sauber Formula 1 test driver was the only man to break through the 1m39s barrier, with his morning best - a 1m38.828s - enough to earn top spot overall.
Several drivers did improve in the afternoon, with Red Bull protege Pierre Gasly the closest to demoting Sirotkin.
The Arden driver worked his way down to a 1m39.013s, leaving him just under two tenths of a second from top spot.
Draco's Pietro Fantin also improved in the afternoon session, and finished the day third overall, just 0.033s down the road from Gasly.
Nikolay Martsenko, Zoel Amberg and Nigel Melker filled the top six on combined times.
Day one pacesetter Oliver Rowland wound up eighth overall in the second Fortec car, while Carlos Sainz Jr - quickest on day two - was 11th, 0.695s off the pace.
Sirotkin caused one of six stoppages as rain and mixed conditions produced a disrupted final day.
Pos Driver Team Time Gap 1. Sergey Sirotkin Fortec 1m38.828s 2. Pierre Gasly Arden 1m39.013s +0.185s 3. Pietro Fantin Draco 1m39.046s +0.218s 4. Nikolay Martsenko Comtec 1m39.190s +0.362s 5. Zoel Amberg AVF 1m39.255s +0.427s 6. Nigel Melker Tech 1 1m39.397s +0.569s 7. Will Stevens Strakka 1m39.423s +0.595s 8. Oliver Rowland Fortec 1m39.456s +0.628s 9. Luca Ghiotto Draco 1m39.458s +0.630s 10. Will Buller Arden 1m39.481s +0.653s 11. Carlos Sainz Jr DAMS 1m39.523s +0.695s 12. Matias Laine Strakka 1m39.602s +0.774s 13. Norman Nato DAMS 1m39.647s +0.819s 14. Matthieu Vaxiviere Lotus 1m39.656s +0.828s 15. Marlon Stockinger Lotus 1m39.759s +0.931s 16. Jazeman Jaafar ISR 1m39.860s +1.032s 17. Tom Dillmann Pons 1m40.026s +1.198s 18. Roberto Merhi Zeta 1m40.040s +1.212s 19. Marco Sorensen Tech 1 1m40.053s +1.225s 20. Meindert van Buuren Pons 1m40.519s +1.691s 21. Roman Mavlanov Zeta 1m40.596s +1.768s 22. Liam Venter Comtec 1m41.289s +2.461s 23. Beitske Visser AVF 1m41.606s +2.778s