Although de Reuver didn't take an MX1 moto win, the popular Honda Martin rider was the most consistent in the wet, heavily rutted, sand - resting on top of a slippery hard packed base - taking 3-3 race finishes from pole position.
Kawasaki's Sebastien Pourcel won race one by six seconds from Suzuki's Ken de Dycker, but the pair were only seventh and sixth respectively in the second race - ending their overall hopes.
Instead it was KTM's Max Nagl who triumphed over home hero David Philippaerts in a hectic final moto - Philippaerts having earlier recovered to a highly impressive fifth in race one after an accident left him at the tail of the field.
The Monster Yamaha rider thus extended his championship lead by finishing a close third overall behind de Reuver and Pourcel, since nearest title rival and reigning world champion, Steve Ramon, was only ninth with 6-9 finishes.
"Someone went down in front of me [in race one] and when I was hit from behind I went over the bike that was lying on the ground. It was a big crash and also disappointing because I knew it would be hard to win the GP after that," said Philippaerts. "I tried as hard as I could. I was watching my lap-times and I knew they were fast; I did the best lap of the race near the end and that was a good thing.
"In the second moto I pushed to get near to Nagl so that he would see me and maybe make a mistake but it was raining a lot and I did not want to make a mistake. I thought of the points and did not take any risks. I wanted to win here but to be on the podium is OK."
In the MX2 class, Cairoli was left at the back of the field after a first lap accident in race one, but fought his way up to a thrilling fourth position by the end of the 18 laps, behind a triumphant Nicolas Aubin, with KTM team-mates Tommy Searle and Tyla Rattray second and third.
But Cairoli didn't make the same mistake twice, leading from the start of race two on his way to a dominant 18secs victory over world championship leader Rattray, with Searle third and fellow Brit Shaun Simpson fourth.
"For sure I wanted to win both races but the second corner [of race one] was a big problem," reflected Cairoli. "Out of the gate I started well but then I had some contact with Tommy. I tried for a good line in the second corner but Eggens was on the inside of me and decided to come across to the outside and hit my front wheel. I lost time restarting the bike but then I pushed hard. I thought that it might still be possible to win the GP so I gave it everything I had. I put my head down and concentrated on good lap-times and catching the next rider. We have had a lot of wet races now and these are not my favourite conditions; although the track was not as slippery as Bulgaria!"
Cairoli thus took the overall by just one point from Rattray and Searle, meaning the trio are separated by just eight points heading into Searle's home event, the British Grand Prix at Mallory Park, on June 1st.