Goals from Ethan Pinnock and Bryan Mbeumo made history for Brentford as they became the first ever Premier League team to win their first three trips to Chelsea, triumphing 2-0.
The Bees also extended their unbeaten run in Premier League London derbies to 13 matches, continuing a club-record sequence, while they have now won four straight away matches in the capital for the first time in their league history.
And to compound Chelsea's woes at Stamford Bridge, the Blues have now won fewer matches at their ground than Brentford since April, with the Bees claiming two successes in that time.
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Chelsea boss Mauricio Pochettino made three changes to the team that had earned an encouraging 2-2 draw with Arsenal, with Axel Disasi, Noni Madueke and Nicolas Jackson starting in place of Malo Gusto, Mkyhailo Mudryk and Enzo Fernandez, with the latter two absent from the squad due to minor injuries.
Chelsea put Brentford under real pressure in the first half, with summer signing Cole Palmer finding pockets of space while the Bees struggled to break through Chelsea's press.
Madueke hit the woodwork from the right side of the box, before Palmer played in Raheem Sterling, whose rushed shot flew high and wide.
But it was a different story in the second half as Brentford made a fast start.
In the 50th minute a low driven cross was met by Vitaly Janelt, who was denied by a strong hand from Robert Sanchez. And then in the 58th minute Mbeumo lofted a cross to the back post, where Pinnock arrived to head powerfully past Sanchez.
Brentford frustrated Chelsea for the rest of the match before grabbing a second goal in the sixth minute of stoppage time.
With Chelsea goalkeeper Sanchez coming forward into the Bees' box for a corner, Brentford broke away and substitute Neal Maupay squared to Mbeumo to gift his team-mate an open goal.
Mbeumo has now scored or assisted five Premier League goals against Chelsea, more than he has against any other side in the competition.
Brentford climb to 10th, moving a point above Chelsea, who have lost their eighth home league match of 2023, their most in a calendar year since 1986.
See: Chelsea report | Brentford report