Adam Flanagan, the Brentwood Town boss, was disappointed in defeat after seeing his side squander several good chances against Thurrock.
Blues were the better side in the first half, with the lively Alex Akrofi hitting the post, but they faded in the second half as Thurrock took control to win the game 3-0.
Ross Wall opened the scoring in the 69th minute, rounding off a slick move with a half-volley rifled across Brentwood keeper Olly Bowls.
An own-goal saw Fleet double the scoring eight minutes later, as Jack Paxman's free-kick from the left ricocheted into the net.
Lamar Reynolds, making his debut as a substitute at half-time, then slotted Thurrock's third in the 89th minute, tapping home after a quick counter-attack - started by the impressive James Goode nicking the ball in the middle.
After the game, Flanagan spoke of his disappointment to the Weekly News and questioned whether Town's stuttering pre-season preparations had proved a hinderance.
Meanwhile, Thurrock boss Mark Stimson was pleased with the win, but reluctant to lavish his players with praise - citing a disappointing first half display as the only negative on an otherwise excellent afternoon for Fleet.
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