The Belgian led both races almost from start to finish in the deep Dutch sand, wining race one by seven-seconds over nearest rival Max Nagl (KTM), before enjoying an eight-second advantage over Billy Mackenzie (CAS Honda) in race two - handing Ken his second ever grand prix win.
"It was a really good day for me. In fact it couldn't have been any better," admitted de Dycker. "It's the first time I've taken a double victory and it's the first time I've ever had the lead in the world championship. I've got to thank the team because they work hard but I also work hard too and this win today is the result of our joint efforts. I had no problems in either race except on the last lap of the second race when I was thinking about winning maybe a little bit too much and my arms pumped up."
Reigning world champion Steve Ramon completed a perfect day for Suzuki with second overall, thanks to his 3-4 race results. Ramon was tied with Nagl on points, but the German's fifth position in moto two gave Ramon the nod.
Mackenzie was fast in both races, but dropped back to tenth in race one after a fall, costing him a place on the overall podium, while factory Yamaha team-mate David Philippaerts and Josh Coppins were fifth and sixth overall.
The pair collided late in race one, sending Coppins - already nursing a foot injury - to the ground, but the world championship contender was able to remount without problem and the Kiwi later claimed to be satisfied with his performance.
"All in all this has been a pretty good weekend under the circumstances," said Josh, who lost the 2007 title due to a shoulder injury. "This weekend was just damage control really and I rode the best I could within the pain, trying not to damage the foot at all. I haven't been able to do the work to be best prepared for this race. I am quite happy with the result which is a relief after what I have gone through in the last two weeks.
"I'm sitting in 6th overall now whereas two weeks ago I was looking at the bones sticking out of my toes so I'm happy with the position I am in. I have the stitches removed from my toes tomorrow and then I plan to get some more time on the bike later in the week. We'll head to France for some guaranteed better weather and hopefully with that time in the saddle I'll be able to build back some strength for Spain and aim to be strong in Portugal," he declared.
In the MX2 class, a confident Tyla Rattray won both races to take his first grand prix victory since Desertmartin in 2006.
The KTM-backed South African led home reigning MX2 champion Tony Cairoli of Yamaha Red Bull De Carli and sand master Erik Eggens of Beursfoon Suzuki.
Rattray's factory team-mate Tommy Searle finished fourth overall after crashing in both motos.