Diss created the first scoring opportunity in the third minute when Charlie Webb managed to get goalside of Jack Wood but, when faced with goalkeeper Brad Cook, he sliced his shot wide of the net.
Next Joe Knight created a chance for Harwich but his cross to the far post came too quickly for the oncoming Alfie Canning to make a clean contact. A little later, the lively Canning also had a goalbound shot deflected for a corner kick.
It was end-to-end in these opening stages with Town's left-back Callum Bray trying a shot that fizzed over the Shrimpers crossbar.
Diss' leading goalscorer Liam Jackson has bagged 17 goals in 18 starts but when he netted with a header, the assistant referee had her flag raised for offside.
Harwich scored what was to prove to be the match-winning goal in the 38th minute when a clever shot on the turn from Alfie Eldridge caused problems in the Diss defence and Louie Bloom (pictured) took advantage to net with a poacher's effort from close range.
It was a highly competitive match that made riveting viewing from start to finish with a determined Diss forcing Harwich to defend at the start of the second half.
Striker Knight was working hard for Harwich and his fierce shot from 25 yards went inches over the Diss crossbar.
Substitute Luke Spurling showed good control when he came on to join the Harwich attack and when Knight and Canning combined he was close to converting the chance created.
With 7 minutes remaining there was mayhem in the Diss goalmouth when Spurling's shot struck the crossbar and then Knight's effort was diverted to Bloom, who seemed certain to score, only for a defender to make a point-blank goal-line clearance.
It was a nail-biting finish and it was Harwich's turn for a great escape when, from right under the crossbar, a Town attacker scooped the best chance of the match over.
In the dying seconds, Spurling created the opening from which Knight's piledriver saw Town's keeper George Howes make a super save to tip the ball over the crossbar.
For Harwich, it was impossible to name a man of the match with every player working their socks off to rise to the challenge that Diss set.
Our thanks to Diss Town Football Club for once again providing a warm, friendly welcome and first-class hospitality.
Harwich team: Brad Cook, Alfie Smith, Jack Wood, Dan Keeble, Reece Harte, Tyler Sullivan, Alfie Eldridge, Greg Rodger, Alfie Canning, Louie Bloom and Joe Knight. Substitutes: Luke Spurling and Elliot Johnson.

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