Nathan Cooper’s first-half header sent Hornchurch into the second qualifying round at the expense of former club Romford at Ship Lane.
Cooper got up to meet Brad Warner’s corner from the left after 18 minutes and it proved enough to earn Urchins the £3,000 in prize money and leave Boro wondering what might have been.
Urchins boss Jimmy McFarlane named an unchanged side from the 4-1 Bank Holiday Monday win at Bridge Avenue, while opposite number Paul Martin made six changes to his starting line-up, having admitted he was trying to “spoof” it in the previous meeting.
But this tie followed the formbook as Bostik North leaders Hornchurch just had enough to see off their neighbours for the second time in six days.
Returning Boro keeper Michael Gooch had to be alert to hold a deflected George Purcell effort on six minutes, after Warner and Ross Wall had combined to set up the shooting chance.
Jay Porter made a strong run forward from left back on 10 minutes, but was snuffed out by James Ishmail and Danny Cossington as he approached the box and a weak cross was comfortably cleared.
But Jonathan Nzengo looked lively on the counter-attack for Boro, only to fall victim to the Hornchurch offside trap, before Purcell was fouled on the edge of the box by Cossington after Tobi Joseph had robbed Jonathan Adebayo of possession about 30 yards out.
Purcell dragged the free-kick horribly wide, though, to give the home side a let-off on 13 minutes.
But there was no escape five minutes later when Cooper met a Brad Warner corner from the left and headed inside the far post to open the scoring against his former club.
Boro’s defence looked at sixes and sevens as Joseph pressed soon after, but Nzengo almost stole in at the far post when a free-kick was played deep into the Hornchurch box on 22 minutes.
Hornchurch were looking the stronger side, though, as the first half wore on, with Martin’s men feeding off scraps of possession and the manager cutting a frustrated figure on the touchline.
And Wall flashed a half-volley behind from a tight angle after Warner and Purcell had helped Sam Mott’s long kick on with half an hour played.
Ben Clarke won a free-kick for Boro in a good position after collecting a nod-down from Chris Taylor, when Gooch sent a long kick downfield.
But Jamie Dicks sent his curling effort comfortably into the hands of Mott and Urchins countered quickly, with Warner playing in Joseph on the right for a shot high over the crossbar and stand behind the goal.
Referee Ryan Head had a long chat with Taylor and Boro captain Adam McGeorge, before Porter went down near the left corner flag after a fairly innocuous challenge from Ishmail.
Porter was able to continue after some treatment and made another good run late in the half, only to well tackled by Ishmail close to the byline.
But McGeorge, who played at the London 2012 Olympics for New Zealand, was booked within two minutes of the restart after catching opposite number Elliot Styles late in midfield.
And Tom Vickers was penalised for holding Wall just outside the Romford box, but Warner’s curling free-kick dropped just over Gooch’s crossbar.
Dicks led a Boro attack on 51 minutes, which saw Chris Taylor’s cross headed over his own crossbar by Jed Chouman, but the corner was cleared and Ben Clarke was tackled by Cooper as he looked to send the ball back into the box.
McGeorge needed treatment soon after, but after a few more minutes had elapsed it was clear he was unable to continue and replaced by Olu Giwa on the hour mark.
Clarke had a shot blocked in the box when a deep cross fell at his feet on 63 minutes, as Boro enjoyed one of their best spells of the tie.
And Martin sent Craig Jeakins on for Clarke for the final 18 minutes, before Hornchurch’s Joseph – only recently recovered from a broken leg – made way for Theo Fairweather-Johnson, whose first contribution was to foul Giwa around 25 yards from the visiting goal.
Dicks fired the free-kick wildly over the crossbar and placed his hands on his face, which said it all, before Gooch saved from Purcell, after pushing a left-wing cross out to the Hornchurch striker.
Warner flashed a dangerous ball across the face of Gooch’s goal, after Wall returned a corner to him on the left, and Fairweather-Johnson flashed a tight-angled chance over the crossbar when Tom Vickers could only head the ball into his path.
Dicks then came off worse after colliding with team-mate Jeakins just outside the Hornchurch box, before Martin sent Nick Reynolds on for Ishmail.
Leon McKenzie replaced Purcell for the visitors and tested Gooch with a well-struck low drive, which the Boro keeper gathered at the second attempt.
And Gooch then fingertipped Warner’s curling free-kick from wide on the right behind for a corner as normal time drew to a close.
Nzengo was crowded out in the box early in stoppage time as Boro began to throw more men forward in a desperate late attempt to force a replay.
And they came close as Cossington got up to meet a corner from the right, but glanced his header beyond the far post.
Romford: Gooch, Ishmail (N Reynolds 85), Atkinson, Cossington, Vickers, Dicks, Adebayo, McGeorge (Giwa 60), Taylor, Clarke (Jeakins 72), Nzengo.
Unused subs: Brown, Burt Allen, Olukoga, S Reynolds.
Hornchurch: Mott, Hursit, Porter, Lee, Cooper, Chouman, Joseph (Fairweather-Johnson 74), Styles, Wall, Purcell (McKenzie 87), Warner.
Unused subs: Tuohy, Hussain, Goodacre, Lambert, Roberts.
Referee: Ryan Head.
Attendance: 333.
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