The Hornets and the Field will meet at the Fusion Aviation Community Stadium on Tuesday night following a 2-2 draw at Woking Park.
Rhys Murphy won and converted a penalty to give Horsham a 26th-minute lead.
But quickfire second half goals from Finn Evans and Kiye Martin saw spirited Westfield, who sit two divisions below the Hornets, turn the tie on its head.
Isaac Philpot spared Horsham’s blushes, scrambling home an equaliser with 12 minutes remaining.
See some of John Lines’ pictures from the game on this page and the ones linked – and you can read all the latest from the Hornets in the West Sussex County Times, out every Thursday.
61. FA Cup: Horsham FC held to replay by valiant Westfield in second qualifying round – the match in pictures
Horsham FC face a midweek FA Cup second qualifying round replay at home to step four Westfield after a stalemate in Surrey. Photo: John Lines
62. FA Cup: Horsham FC held to replay by valiant Westfield in second qualifying round – the match in pictures
Horsham FC face a midweek FA Cup second qualifying round replay at home to step four Westfield after a stalemate in Surrey. Photo: John Lines