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Ian Pulman scored his first hat-trick for in-form Whitstable to end Ashford's unbeaten 15-game run.
They dashed the hopes of Steve Lovell's side going top of Ryman League Division 1 South with a 4-1 win at Belmont on Tuesday night.
Pulman put Whitstable ahead after only three minutes, and Danny Tipple doubled the lead 13 minutes later - with only his second goal in 76 games - before Craig Wilkins pulled one back just after the half-hour.
Pulman made it 3-1 after 48 minutes, again set up by Tipple, who then provided the assist for his treble after 63 minutes. Marcos Perona almost added a fifth but was denied by the woodwork.
Elsewhere, two Kent Senior Cup quarter-finals took place.
Bromley progressed to the last four after a 2-0 victory at Maidstone thanks to a goal each from Ryan Hall and Warren McBean.
Ramsgate's tie with Thamesmead - previously abandoned due to floodlight failure - went the distance with the hosts winning 3-2 after extra-time.
Tom Tsangarides put Ramsgate ahead in the 39th minute but Thamesmead equalised through Peter Smith three minutes before the break.
Smith put the Mead ahead before a Rikki Cable penalty was saved by Ramsgate keeper Shaun Alliston - the key turning point of the game.
Ryan Royston equalised in the last minute of normal time before Tsangarides got his second of the night after 102 minutes.
The sides join Welling in the last four with the remaining quarter-final between Whitstable and Ebbsfleet due to be played on Wednesday, February 25.