Visser moved up to second overall, on 37 points and six behind Chadwick, in the series which uses identical Formula Three cars.
"They tried to make it hard for me but starting from P2 (second place), I had a perfect start," said Visser. "I think we did a perfect job today."
There was drama at the start with Belgian driver Sarah Bovy lining up out of position and with smoke billowing from her stranded car.
Finnish driver Emma Kimilanen had already dropped out on medical advice, after a crash in Hockenheim triggered the recurrence of an old injury, and was replaced by Hungarian reserve Vivien Keszthelyi.
The safety car was deployed briefly after the opening lap and Visser then pulled away at the restart ahead of Chadwick and Powell.
Another safety car period came after Britain's Esmee Hawkey tried a move up the inside of Polish driver Gosia Rdest and the two collided.
The race settled down with 15 minutes remaining and Visser continuing to increase the gap to Chadwick, coming under increasing pressure from Powell.
Powell seized second place at turn nine with two minutes remaining, and the two Britons running wheel to wheel, but Chadwick went back in front with a lap to go.
The six-race series is aimed at helping women up the motorsport ladder. The overall winner will collect $500,000, with prize money down to 18th place.
(Writing by Ian Page, editing by Alan Baldwin and Clare Fallon)
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