Horsham's Ryan Rummery struck after just 14 seconds on his way to a superb hat-trick as Leatherhead were sunk 4-1.
Rummery's quickfire effort after cutting in from the right got Hornets off to a flyer but Wes Harrison levelled on five minutes.
An overhead kick followed by a solo run and strike completed Rummery's treble on his first start of the season and Lee Carney wrapped up victory from the spot after Florian Mateos was brought down.
Hornets boss John Maggs said: "The second-half performance was our best of the season. Ryan takes the headlines and he has given us another dimension up front but Flo supported him really well."
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