Bogey side continues to haunt Blues
But unlike in the 3-0 and 5-0 reverses earlier in the season, The Blues did at least run their Sussex Division One rivals close on a soggy Saturday afternoon.
And they could even have taken this last eight tie to extra-time had they made more of a late three-on-one opportunity.
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Hide AdJoint manager Andy Laskey said: "It deserved to go to extra-time. For the first time this season, we actually competed with them and that made the big difference."
Two goals in the space of six minutes just after the hour ultimately cost them a last four showdown against Hassocks or league leaders Whitehawk.
After being the better side during the early part of the second period, Sidley fell behind in the 62nd minute when Scott French drilled a fine effort into the bottom left-hand corner.
One became two in somewhat fortunate fashion, a shot from Sussex football's leading scorer, Dan Leach, taking a wicked deflection off Nick Whiteman and looping agonisingly over Peter Newstead.
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Hide AdIn between, Dave Henham missed a golden opportunity to equalise by firing over on the volley from only a few yards out.
Graham Morris did find the net 10 minutes from time, heading home a free kick by Adam Day which had been flicked on.
And United almost pulled the game out of the fire but Scott McDonald neither shot, nor passed with the goal at his mercy.
The first half ended goal-less, a scenario for which Sidley had Newstead to thank because he denied the dangerous Leach on two occasions.