Lennon Docherty took victory by a slim 0.007secs over Jacob Stephenson after an exciting 16-lap race one.
The delighted youngster held off his rival to take the spoils as Jack Kirsch crossed the line third.
Finn Smart-Weeden was fourth, just a further 0.194secs behind Kirsch as Lewis Jones took fifth.
HEL Performance British Junior Supersport Championship with Motocourse, race one:
1. Lennon Docherty (Symcirrus Motorsport)
2. Jacob Stephenson (Kevin Stephenson) +0.007s
3. Jack Kirsch (RD Racing) +0.176s
4. Finn Smart-Weeden (Wrapped Racing by Club 74) +0.370s
5. Lewis Jones (Symcirrus Motorsport) + 2.276s