Two tries inside the last 10 minutes saw Harpenden’s steely hold on a play-off place broken by their biggest challengers.
Chiswick’s visit to Redbourn Lane looked like extending Harpenden’s advantage to 10 points and all but guaranteeing them a shot at promotion to the next level.
As it is the 19-15 loss means everything is back up in the air with the west London side now just four points behind and with a game in hand.
Skipper Oli Lacey said: “We’re bitterly disappointed as we thought we’d done enough to win but we turned off at key moments.
“We must now draw a line under the result and move on to the next game.”
The match was as tense and hard-fought as expected and even though Harpenden failed to register a try for the first time this season, five penalties from the dependable boot of Tom Sweeney had their noses in front right up until the latter stages.
He began his kicking masterclass as early as the third minute but the hosts were then dealt hammer blows when they lost giant tight-head prop Sam Staff and powerful lock Al Whittaker to injury, although a heavily patched up Whittaker was able to return to the fray in the second half.
Fortunately for Harps ill-discipline by Chiswick allowed Sweeney to convert two more penalties on 19 and 32 minutes for a 9-0 lead.
Those weren’t the final points of the half though as the visitors continued to serve it up and were eventually rewarded when right wing Adam Churton went over for a try five minutes before the break.
And Harps’ injury curse continued before they reached the interval when flanker Tom Sanders was forced off.
They still managed to keep the scoreboard ticking over though and two further kicks from Sweeney on 53 and 56 minutes gave the hosts a 10-point an advantage.
It was a lead that looked to be giving them the vital success but with 10 minutes to go Chiswick flanker Ryno Lopez went over for a score under the posts that was converted by Oliver Snooks.
And with the scent of victory in their nostrils, the Londoners threw everything at Harps, finally finding a chink in the armour with just two minutes remaining thanks to Snooks.
The conversion by George Bibby was merely an exclamation point.
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