Brentford stunned Stamford Bridge again, winning there for the third season in a row.
Ethan Pinnock put the Bees in front and Bryan Mbeumo’s stoppage-time goal sealed another west London derby triumph for Thomas Frank’s side.
Chelsea were wasteful and were then punished for some slack defending just before the hour mark, when Pinnock headed in Mbeumo’s cross.
And when Blues keeper Robert Sanchez went up for a corner in the final moments as the hosts chased an equaliser, they were hit on the counter-attack.
Brentford cleared the ball as far as Ian Maatsen, who was beaten by Mbeumo, resulting in Neal Maupay racing through on goal.
Sanchez sprinted back and challenged Maupay but the striker managed to tee up Mbeumo for a simple finish.
Chelsea were booed off by their fans after a ninth home defeat this year.
They paid the price for failing to make their first-half possession count – Noni Madueke hit the post but Brentford defended well and their keeper Mark Flekken was barely tested.