Harwich started well, creating chances, and were eventually rewarded midway through the first half when Jeremiah Ajisafe (pictured) gave them the lead.
It was to prove a remarkable opening 45 minutes for the Shrimpers' central defender as he scored twice more to complete his hat-trick and give his team a 3-0 interval lead.
Hadleigh had threatened on a couple of occasions, firstly when Shrimpers goalkeeper Jack Lines blocked a fierce drive and later he thwarted them again when he came out to dive at the feet of an attacker.
United scored twice within the first 15 minutes of the second half to narrow the score to 3-2, their second goal being a cracking strike that gave Lines no chance of stopping.
However, Harwich were in no mood to allow the comeback to continue.
Dylan Crowley restored their 2-goal advantage with a really well-taken goal, shooting from wide on the right flank, low into the far corner of the net.
The Shrimpers' 5th goal was equally impressive, Josh Beavis shooting from a acute angle, the ball similarly nestling in the bottom of the net.
It was an excellent all-round team performance from Harwich that had their manager, Luke Harrison, happily looking forward to Tuesday night's local derby with Wivenhoe Town Under-23s under the Royal Oak floodlights, kick-off 7.45pm.

Harwich team: Jack Lines, Harry Hosking, Alex Pryke, Theo Stevenson-Miller, Jeremiah Ajisafe, Rory Breen, Josh Beavis, Hary Kabangu, Dylan Crowley, Rob Pratt and Ewan Livingston. Substitutes: Jinghan Qi, Cody Harrison and Aidan James.