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Lacklustre Dartford tasted defeat for the first time this season in National League South on Bank Holiday Monday.
The Darts never really got going against an East Thurrock side who remain unbeaten and maintained their place in the top three courtesy of Sam Higgins' first-half goal.
Dartford improved after the break but they were unable to force home an equaliser.
Dartford were without keeper Deren Ibrahim, who is on international duty with Gibraltar, so in came on-loan Charlton teenager Jordan Beeney.
Chances were at a premium during a cagey first half when both sides cancelled each other out for long periods.
Dartford too often went long and lacked support to their front men while Ryan Hayes cut a frustrated figure down the right and Lyle Della-Verde was caught in possession too often on the other flank.
Lee Noble shot over from distance and while Darts forced a couple of corners it was the visitors who found the breakthrough on 21 minutes.
Reece Harris' free-kick seemed to catch out Beeney at his near post, the ball being diverted onto the bar by the keeper before Higgins - who was being marked by Hayes prior to the kick being taken - was on hand to convert from close range.
It was East Thurrock's only noteworthy attack in the first half. It was the amount of time that Harris and then Lewis Smith were down getting treatment that angered the home fans, their frustration compounded when referee David Spain only added one additional minute.
Darts' best opening came five minutes before the break as the willing Warren Mfula chased a long ball down, he beat keeper Lukas Lidakevicius but his effort trickled wide of the empty goal.
The second half proved a much better spectacle, although the home side never really got going.
Alfie Pavey nearly forced home a long throw-in and Hayes shot wide while Higgins twice went close for the visitors.
Darts went close to an equaliser in the 69th minute. Pavey showed neat footwork to cut inside two defenders but keeper Lidakevicius did well to beat hit shot away to safety.
Dartford piled on the pressure in the second half, Harris, substitute Alfie Cue and Elliot Bradbrook were all denied while penalty appeals were turned down.
Then when a clear chance did come their way, substitute Andy Pugh headed Harris' cross straight into the keeper's hands.
Dartford: Beeney, Harris, Onyemah, Bonner, Sho-Silva (Wood 62mins), Noble, Hayes, Bradbrook, Pavey, Mfula (Pugh 56mins), Della-Verde (Cue 72mins). Subs not used: Ajala, Brown.
East Thurrock: Lidakevicius, Peddie (Sammons 57mins), Harris, Allen, Ekpiteta, Burns, Agyemang (Ruel 61mins), Cornhill, Higgins, Akinwande (Honesty 85mins), Smith. Subs not used: Boswell, White.
Attendance: 934.