A last minute goal by Shawn Mitchell condemned St Leonards to a 2-1 defeat at Chatham Town in the Dr Martens League eastern division.
The game looked destined to finish level until Mitchell fired home to condemn Saints to a sixth consecutive league defeat.
The visitors had taken the lead after just four minutes when Paul Wilson fired home from close range after a flick by Sean Ray.
Their advantage lasted only three minutes when Chatham equalised through Garry Tilley who was left with an easy tap in after Brad Poole had overhit a back pass.
Saints striker Ray had to deputise in goal for the last ten minutes after keeper Peter Newstead was carried off injured.
Saints caretaker-boss Terry White said: "I'm absolutely gutted. I couldn't have asked for any more from the players in terms of commitment and effort, but unfortunately that doesn't always win you games."
Elsewhere in Sussex, only two games survived the weather. Both came in the Sussex RUR Cup, with Chichester claiming a spot in the quarter finals after beating Sidlesham 3-2 at Church Road.
A gripping local derby saw Scott Tipper head the visitors into a third minute lead.
Neil Murfin, having his first start up front, equalised for Chi three minutes later and then Jamie Laidlaw put the hosts 2-1 ahead. However, Richard Davies squared the match just before half time.
In the second half Chi hit the bar through Paul Jones' 30 yarder and the post via Laidlaw's shot while Alex Ward had an effort disallowed.
Just before the end, though, Ben Hitchman headed home a free kick to clinch victory.
Assistant-manager Gary Brockway said: "It was a tough cup tie. Both teams gave 100 per cent commitment and there was some good football along the way."
Wealden caused an upset as they went through 14-13 on penalties after drawing 0-0 with Hailsham Town.
Lee Sale and Graham Leach were Wealden's heroes. Sale hit the winning penalty while Leach palmed one of Hailsham's spot kicks over the bar.
Wealden boss Gary Allen said: "We are well pleased. We have come here, worked hard and got a result."
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