Nico Muller set the pace in a rain-hit second day of Formula Renault 3.5 testing at Barcelona on Wednesday.
The Draco driver set a best of 1m48.116s early in the morning session, and the conditions made it impossible for the field to beat him through the remainder of the day.
Kevin Magnussen, quickest on the opening day of testing, was second fastest in the Carlin car, less than two tenths of a second behind Muller. Marco Sorensen, Andre Negrao and Robin Frijns completed the top five - their best times all set in the morning. Richie Stanaway set the pace in the afternoon, but his time was nearly two seconds slower than Muller's.
"The new treads are at their best when there's a lot of water on the track," said Muller. "They offer an amazing amount of grip in extreme conditions.
"We need to learn how to look after these tires, though, because they get worn really quickly if you're not careful and then you're in whole lot of trouble. We've still got a lot of work to do in the dry, but having the chance to work on a wet track has given us some useful preparation for the season ahead."
The FR3.5 teams will return to testing action at Motorland Aragon on April 18-19.
Pos Driver Team Time
1. Nico Muller International Draco Racing 1m48.116s
2. Kevin Magnussen Carlin 1m48.246s + 0.130
3. Marco Sorensen Lotus 1m48.337s + 0.221
4. Andre Negrao International Draco Racing 1m48.613s + 0.497
5. Robin Frijns Fortec Motorsports 1m49.186s + 1.070
6. Vittorio Ghirelli Comtec Racing 1m49.222s + 1.106
7. Richie Stanaway Lotus 1m49.280s + 1.164
8. Jake Rosenzweig Isr 1m49.388s + 1.272
9. Sam Bird Isr 1m49.411s + 1.295
10. Alexander Rossi Arden Caterham 1m49.530s + 1.414
11. Giovanni Venturini Bvm Target 1m49.552s + 1.436
12. Nick Yelloly Comtec Racing 1m49.589s + 1.473
13. Jules Bianchi Tech 1 Racing 1m49.720s + 1.604
14. Nicolay Martsenko Bvm Target 1m49.835s + 1.719
15. Kevin Korjus Tech 1 Racing 1m50.024s + 1.908
16. Carlos Huertas Fortec Motorsports 1m50.077s + 1.961
17. Arthur Pic Dams 1m50.130s + 2.014
18. Will Stevens Carlin 1m50.158s + 2.042
19. Mikhail Aleshin Team Rfr 1m50.185s + 2.069
20. Daniil Move P1 Motorsport 1m50.666s + 2.550
21. Lewis Williamson Arden Caterham 1m50.900s + 2.784
22. Anton Nebylitskiy Team Rfr 1m50.943s + 2.827
23. Walter Grubmüller P1 Motorsport 1m50.953s + 2.837
24. Zoel Amberg Pons Racing 1m51.117s + 3.001
25. Lucas Foresti Dams 1m51.122s + 3.006
26. Yann Cunha Pons Racing 1m52.296s + 4.180