As the highest ranked privateer on the World Championship tour, Simpson was an unfamiliar face on the top step of the podium having won just one GP in his career.
However, he dominated on the sandy dunes of Lommel, one of the gnarliest tracks of the season, to take the victory in both races for the first time.
He saw off competition from tour leader Romain Febvre, who suffered a number of crashes before finishing with a double third place, while Gautier Paulin finished second overall.
Simpson is now just four points off the top five, as he looks to break into the elite ranks, but it could have been so different had he not held on after a dramatic crash with two laps to go of his second moto.
“It was a bit of a stupid one.” he said. “I don’t think I’ve ever got up as quick as I did then.
“This is probably the roughest I’ve seen Lommel, but I would call this real motocross, plenty of bumps, a lot of places to pass and a great contrast to what we are used to. It was totally enjoyable.”
Anstie was also celebrating a double victory but he sits in seventh in the overall standings while Pauls Jonass became the red plate holder in the absence of Jeffery Herlings after finishing second.