Jamie Taylor struck the second half winner which sets up a local derby final for Horsham in the Sussex Floodlight Cup.
They beat Burgess Hill 1-0 at Littlehampton in last night's semi and will now face neighbours Horsham YMCA or old rivals Crawley Town.
Hornets had the better of the chances in the opening exchanges. After five minutes, Taylor's effort was blocked by Hillians' keeper Pat Gannon and Steve Payne fired the rebound narrowly wide.
Gannon was in action again on the half-hour, pushing a Luke Gedling free kick round the post and he held a Taylor header from Gary Charman's cross moments later.
At the other end, Hornets' keeper Ian Chatfield had to be sharp to save at the feet of Ashley Carr and he made another fine block from a Nicky Sullivan effort.
The breakthrough came five minutes after the break. Gannon blocked Marcel Dennis's volley and Taylor knocked in the rebound.
Hornets continued to press and Gannon made saves from Charman, Payne and Taylor.
Horsham's Dave O'Donnell was sent off a quarter-of-an-hour from time for a second booking.
Daren Newman almost capitalised on a rare defensive error but Horsham held on.
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