Harwich were without goalkeeper Will Healy through injury so Sam Felgate (pictured) came into the team for his first appearance of the season.
The Shrimpers came close to scoring in the first minute when Basildon goalkeeper Mackenzie Drew just managed to save Jake Gordon's shot with his leg.
Harwich continued to do well in the early stages with Liam White exchanging passes with Jeremiah Ajisafe before having his goalbound shot diverted for a corner kick.
With half an hour gone, United were defending again and this time were fortunate when another deflection from a powerful Josh Hawkins shot could have ended up in the net, but instead resulted in a corner.
Two minutes later, it was Harwich's turn to have a narrow escape when a defensive mix-up ended with a shot going inches past the far post.
Gordon created two more openings before the half-time whistle, firstly with Ajisafe just unable to connect with his clever pass and then, from his cross, Louie Bloom bullet-heading the ball off target.

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Only 2 minutes of the second half had elapsed when Spiros Lagopodis slid the ball into the net from an accurate cross to give Basildon the lead.
Then, in the 65th minute, United took a firm grip on the game with a second goal netted by Robbie Allen.
It was now essential for Harwich to get the next goal if they were to have any chance of getting something from the match, and Gordon did fire in a dangerous corner-kick that United did well to clear.
However, Basildon were exploiting Harwich's vulnerability to pacy counter attacks on the rain-saturated surface and from one such move, Chinedu Duru scored their third goal which put the result beyond doubt.
In the final seconds of the match, the Shrimpers were pushing forward when they lost possession on the halfway line, leaving themselves wide open for Lagopodis to score his second and United's fourth goal.
Harwich team: Sam Felgate, Rory Harman, Charlie Canham, Lewis Wood, Jeremiah Ajisafe, Jenson Goode, Alfie Eldridge, Josh Hawkins, Liam White, Louie Bloom and Jake Gordon. Substitutes: Alfie Canning, Tiwa Odufuye, David Ejakita and Munashe Chirimuuta (all used).