Sergey Sirotkin topped the additional Formula Renault 3.5 test session at Monza on Thursday afternoon ahead of this weekend's season opener.
The Russian driver moved to the top of the times just as rain started to fall at the far end of the circuit, affecting the Lesmo corners in particular.
The ISR driver's lap edged him ahead of Arthur Pic, who had gone fastest for new team AV Formula just before that.
Will Stevens had enjoyed a spell at the top of the times before Sirotkin and Pic got their laps in as the first drops of rain fell, and the Briton ended the day third fastest for P1 Motorsport.
Stevens ended up a few tenths of a second shy of the top two, but he was at the head of a very closely bunched group, just edging out pre-season favourite Antonio Felix da Costa and reigning Formula Renault Eurocup champion Stoffel Vandoorne.
Thursday's session was the first of two additional tests being given to teams this season as a result of the cancelled day of running at Aragon in February due to snow.
The teams will get another three-hour session on the eve of the second round of the series at the track where the original test took place.
Pos Driver Team Time Gap Laps 1. Sergey Sirotkin ISR 1m33.817s 39 2. Arthur Pic AV 1m33.832s + 0.015s 39 3. Will Stevens P1 1m34.548s + 0.731s 38 4. Antonio Felix da Costa Arden Caterham 1m34.565s + 0.748s 39 5. Stoffel Vandoorne Fortec 1m34.579s + 0.762s 49 6. Marco Sorensen Lotus 1m34.617s + 0.800s 48 7. Oliver Webb Fortec 1m34.692s + 0.875s 43 8. Kevin Magnussen DAMS 1m34.708s + 0.891s 40 9. Lucas Foresti Comtec 1m34.955s + 1.138s 43 10. Marlon Stockinger Lotus 1m35.059s + 1.242s 52 11. Christopher Zanella ISR 1m35.117s + 1.300s 37 12. Carlos Huertas Carlin 1m35.153s + 1.336s 39 13. Matias Laine P1 1m35.157s + 1.340s 29 14. Daniil Move Comtec 1m35.180s + 1.363s 41 15. Nigel Melker Tech 1 1m35.294s + 1.477s 32 16. Nico Muller Draco 1m35.307s + 1.490s 47 17. Pietro Fantin Arden Caterham 1m35.351s + 1.534s 38 18. Jazeman Jaafar Carlin 1m35.411s + 1.594s 48 19. Andre Negrao Draco 1m35.433s + 1.616s 38 20. Yann Cunha AV 1m35.454s + 1.637s 45 21. Mihai Marinescu Zeta 1m35.496s + 1.679s 41 22. Mikhail Aleshin Tech 1 1m35.535s + 1.718s 45 23. Norman Nato DAMS 1m35.819s + 2.002s 38 24. Zoel Amberg Pons 1m36.051s + 2.234s 42 25. Nikolay Martsenko Pons 1m36.305s + 2.488s 34 26. Emmanuel Piget Zeta 1m36.492s + 2.675s 39