BRIGHTLINGSEA REGENT saw their lead at the Ryman League division north summit trimmed to three points after losing at home to Thurrock.
The R's were pipped 1-0 - their first defeat since Boxing Day - while second-placed MALDON AND TIPTREE closed the gap by winning 3-1 at home to Aveley.
Thurrock are now third in the table, nine points Regent but with a game in hand, while AFC Hornchurch sit fourth after losing 2-1 to Cheshunt.
Bowers and Pitsea occupy the final play-off place after a 5-0 demolition of Ware.
Perry Dunn's 38th-minute goal settled a tense, hard-fought contest at North Road, converting at the far post after a cross from the left.
But in a game of few chances, Regent went close to levelling after the break when Robbie Rees was denied from a one-on-one opportunity and then, from the rebound, Connor Hogan's powerful effort smashed into team-mate Tom Richardson.
Richardson also went agonisingly close with a header that came back off the bar.
Brightlingsea had gone 14 games unbeaten, including winning their last seven, since losing at Dereham Town over Christmas.
And Previous leaders Maldon took full advantage of Regent's defeat as they saw off Aveley, now managed by former Jammers boss Terry Spillane.
Liam Nash was their undisputed star, blasting a hat-trick in the 3-1 victory at Park Drive.
His opener came as early as the second minute, when he lobbed the keeper from a tight angle.
He then added two second-half penalties - the second of which came shortly after Montel Agyermang's unfortunate own goal had made it 2-1.
Aveley's Michael Toner was sent off after collecting two yellow cards.
The Ryman League programme had got underway on Friday night, with WITHAM TOWN sinking near neighbours HEYBRIDGE SWIFTS 5-3.
A bumper crowd of 312 were in attendance at the Village Glass Stadium and they saw Adam Flint's side go three-up inside 38 minutes, following goals from Ryan Charles, Tim Monsheju and Nathan Koranteng.
Reece Grant pulled one back for Swifts but Witham netted twice more early in the second half through Damien Green and Charles again.
It left Heybridge staring down the barrel at a big defeat, but they reduced the deficit thanks to late strikes from George Cole and Adam Adeoye Yussef.
Witham are 14th in the table and Swifts 19th, although the latter have at least one game in hand on most of the sides around them and they play on Tuesday night, at home to Wroxham.