Sam Bird led the way for ISR in Formula Renault 3.5 free practice at Spa on Friday.
The Mercedes Formula 1 test driver, who took the lead of the championship with victory last week in Monaco, moved to the top of the times early in the session and stayed there until the end.
The Briton's time came under attack in a final flurry of activity, as several cars improved in a seven-minute burst which came after a long red flag following a heavy crash for Arden's Alexander Rossi.
Richie Stanaway, who pipped Rossi to top spot in the morning session, got closest for Lotus, knocking long-time second fastest man Kevin Magnussen (Carlin) back to third. The Dane was one of the few drivers in the top 10 - along with Bird - not to improve his time at the end.
Marco Sorenson rounded off a good day for Lotus by taking fourth, while Kevin Korjus, who had been on the fringes of the top 10 with Tech 1 team-mate Jules Bianchi, jumped into contention in the final minutes and ended up fifth quickest.
Daniil Move (P1) and Nico Muller (Draco) improved a couple of times on their final runs to take sixth and seventh, which completed former championship leader Robin Frijns' fall from fourth to eighth for Fortec in the final few minutes.
Afternoon times: Pos Driver Team Time Gap 1. Sam Bird ISR 1m58.838s 2. Richie Stanaway Lotus 1m58.869s + 0.031s 3. Kevin Magnussen Carlin 1m58.878s + 0.040s 4. Marco Sorensen Lotus 1m58.895s + 0.057s 5. Kevin Korjus Tech 1 1m58.901s + 0.063s 6. Daniil Move P1 1m59.068s + 0.230s 7. Nico Muller Draco 1m59.071s + 0.233s 8. Robin Frijns Fortec 1m59.113s + 0.275s 9. Arthur Pic DAMS 1m59.174s + 0.336s 10. Mikhail Aleshin RFR 1m59.286s + 0.448s 11. Carlos Huertas Fortec 1m59.384s + 0.546s 12. Walter Grubmuller P1 1m59.409s + 0.571s 13. Will Stevens Carlin 1m59.585s + 0.747s 14. Nick Yelloly Comtec 1m59.595s + 0.757s 15. Jake Rosenzweig ISR 1m59.623s + 0.785s 16. Jules Bianchi Tech 1 1m59.626s + 0.788s 17. Lucas Foresti DAMS 2m00.173s + 1.335s 18. Alexander Rossi Arden 2m00.377s + 1.539s 19. Andre Negrao Draco 2m00.619s + 1.781s 20. Giovanni Venturini BVM Target 2m00.738s + 1.900s 21. Zoel Amberg Pons 2m00.812s + 1.974s 22. Lewis Williamson Arden 2m00.975s + 2.137s 23. Yann Cunha Pons 2m01.091s + 2.253s 24. Vittorio Ghirelli Comtec 2m01.120s + 2.282s 25. Anton Nebylitskiy RFR 2m01.385s + 2.547s 26. Nikolay Martsenko BVM Target 2m01.528s + 2.690s Morning times: Pos Driver Team Time Gap 1. Richie Stanaway Lotus 1m59.441s 2. Alexander Rossi Arden 1m59.769s + 0.328s 3. Sam Bird ISR 1m59.877s + 0.436s 4. Nico Muller Draco 1m59.949s + 0.508s 5. Kevin Magnussen Carlin 1m59.985s + 0.544s 6. Kevin Korjus Tech 1 1m59.995s + 0.554s 7. Carlos Huertas Fortec 2m00.024s + 0.583s 8. Arthur Pic DAMS 2m00.050s + 0.609s 9. Daniil Move P1 2m00.188s + 0.747s 10. Jules Bianchi Tech 1 2m00.369s + 0.928s 11. Robin Frijns Fortec 2m00.377s + 0.936s 12. Anton Nebylitskiy RFR 2m00.492s + 1.051s 13. Nick Yelloly Comtec 2m00.510s + 1.069s 14. Marco Sorensen Lotus 2m00.578s + 1.137s 15. Jake Rosenzweig ISR 2m00.698s + 1.257s 16. Walter Grubmuller P1 2m00.722s + 1.281s 17. Vittorio Ghirelli Comtec 2m00.992s + 1.551s 18. Will Stevens Carlin 2m01.143s + 1.702s 19. Zoel Amberg Pons 2m01.267s + 1.826s 20. Andre Negrao Draco 2m01.287s + 1.846s 21. Mikhail Aleshin RFR 2m01.320s + 1.879s 22. Giovanni Venturini BVM Target 2m01.385s + 1.944s 23. Lewis Williamson Arden 2m01.428s + 1.987s 24. Yann Cunha Pons 2m01.618s + 2.177s 25. Nikolay Martsenko BVM Target 2m02.086s + 2.645s 26. Lucas Foresti DAMS 2m08.678s + 9.237s