Constant rain made the Supercross-style sand-based circuit very slippery and rutted, conditions which seemed to suit de Reuver.
The Dutchman edged out Yamaha's home hero and world championship leader Davide Philippaerts by a comfortable 0.6secs, with Suzuki's world champion Steve Ramon over one second from pole in third.
Another Suzuki rider, Clement Desalle, and Kawasaki's Tanel Leok will have fourth and fifth gate picks, while Ken de Dycker (Suzuki), Sebastien Pourcel (Kawasaki), Kornel Nemeth (KTM), Maxl Nagl (KTM) and Jonathan Barragan (KTM) complete the top ten.
Two high profile victims of the testing track conditions were Philippaerts's team-mate Josh Coppins (16th) and CAS Honda's Billy Mackenzie (14th).
In MX2, Yamaha's double world champion and home hero Antonio Cairoli won the first qualifying heat by 8.7secs over Molson Kawasaki's Stephen Sword, while Tommy Searle beat factory KTM team-mate and current world championship leader Tyla Rattray by 6.5secs in race two.
The weather forecast for tomorrow suggests constant rain while the ambient air temperature should be in the region of 22 degrees Celsius.
