VIDEO: Sunday Rocks are in the final - but only just
Karl Westbrook got the extra-time winner as a penalty shoot-out loomed.
Click here to see a video of the dramatic semi-final
Avisford's Ian Anthony broke free after just two minutes to slot home.
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Hide AdOn 31 minutes, the Rocks pulled level through an unexpected source as Gary Shaw crossed the ball in for Deano to head home inside the six-yard box.
It was 2-1 to the Rocks nine minutes later. Jamie Bainer was upended in the box, the ref played a superb advantage and Westbrook slotted home.
Avisford took advantage of a strong wind behind them at the start of the second half when Anthony claimed his second on 51 minutes.
Avisford took the lead when a bouncing cross was headed into his own net by Jamie McCulloch and they had one foot in the final on 74 minutes when Anthony completed his hat-trick.
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Hide AdIn the 88th minute, Cooper rifled home a long-range free-kick to make it 4-3.
Three minutes into injury time Dean Bennett was fouled and Westbrook converted the spot kick to set up extra-time.
The same player's stunning volley won it with minutes left.
Westbourne or BFC Felpham await in the final.