Kazim Vasiliauskas topped Friday's final Formula 2 free practice session at Valencia.
The Lithuanian set the best time of the half hour in the first five minutes, just as the weather turned.
Rain persisted for the rest of the session so there were no improvements up front, but the changing conditions did make for some splits in strategy.
Championship leader Dean Stoneman stayed out pounding around on slicks as the circuit got wetter, while everyone else headed for the pits.
Most drivers emerged on wets, though Sergei Afanasiev, Stoneman, Benjamin Bailly, Armaan Ebrahim and series newcomer Ramon Pineiro kept their slicks on.
Stoneman and title challenger Jolyon Palmer ended up on track together with differing strategies. Palmer was initially faster on his wets before Stoneman began to pull clear on slicks - though the laptimes were by then 15 seconds shy of the dry pace.
Stoneman ended up second fastest, with Afanasiev third and Ivan Samarin, Will Bratt and Kelvin Snoeks completing the top six. Pineiro finished his first day in F2 in 10th.
Pos Driver Time Gap Laps 1. Kazim Vasiliauskas 1m28.466s 6 2. Dean Stoneman 1m29.211s +0.745 14 3. Sergei Afanasiev 1m29.316s +0.850 11 4. Ivan Samarin 1m29.409s +0.943 6 5. Will Bratt 1m29.431s +0.965 10 6. Kelvin Snoeks 1m29.513s +1.047 8 7. Nicola de Marco 1m29.592s +1.126 5 8. Benjamin Bailly 1m29.791s +1.325 12 9. Ricardo Teixeira 1m29.918s +1.452 5 10. Ramon Pineiro 1m29.980s +1.514 8 11. Philipp Eng 1m30.036s +1.570 7 12. Jack Clarke 1m30.657s +2.191 11 13. Benjamin Lariche 1m30.695s +2.229 10 14. Armaan Ebrahim 1m30.698s +2.232 9 15. Natalia Kowalska 1m31.054s +2.588 4 16. Johannes Theobald 1m31.151s +2.685 12 17. Mihai Marinescu 1m31.403s +2.937 14 18. Plamen Kralev 1m32.260s +3.794 10 19. Jolyon Palmer 1m40.941s +12.475 9