The 23-year-old Belgian held off local sand specialist Marc de Reuver by 0.3secs on the soft and heavily rutted surface, with new Monster Yamaha signing David Philippaerts a further two tenths back.
"I had a good lap in the beginning of the session but then a few riders went a little quicker than me so I had to go out again," said de Dycker. "It was maybe not until the last lap that I took pole because I crashed and made some small rider errors. It was a tough session, the track was difficult today and it was easy to make mistakes so I'm very pleased to be fastest.
"Now that I'll have first pick of the gate that will be a big help because the start will be very important here. I hope the weather will be good because the track is not so much fun like this. If I can do two 40-minute motos at full speed and without any problems then the result will be there I think," he concluded.
CAS Honda's Billy Mackenzie was fourth fastest with de Dycker's team-mate and reigning world champion Steve Ramon fifth.
"The track is difficult and there are many long deep ruts on the face of the jumps. These caused me to crash on the finish line jump when my front wheel took one rut and my rear wheel took another," explained Steve. "The start will be very important tomorrow because there were nearly five guys who were very close on times during qualification today. I'm feeling confident, my riding is going good at the moment and I hope to win or at least be on the podium."
Manuel Priem (Kawasaki), Max Nagl (KTM), Josh Coppins (Yamaha), Kevin Strijbos (Kawasaki) and Jonathan Barragan (KTM) completed the top ten.
In the MX2 class, Tyla Rattray beat reigning champion Antonio Cairoli for victory in the first qualifying race, while Rattray's KTM team-mate Tommy Searle dominated race two.
