Brentford snatched a 2-0 win away at Chelsea on Saturday thanks to a header by defender Ethan Pinnock and a last-minute breakaway goal by Bryan Mbeumo as the expensively assembled Blues failed to turn possession into goals once again.
Pinnock, undetected by Chelsea's defence, connected powerfully with an Mbeumo cross in the 58th minute, stunning the hosts who had spent much of the game buzzing around the Bees' penalty area but without creating many clear chances.
As the Blues poured forward in the 96th minute in search of a late equalizer, goalkeeper Robert Sanchez ventured up for a corner only to be caught out when the visitors broke away and Mbeumo scored into an empty net.
Chelsea have now won only one of their last 13 home league games, a dismal run stretching back into last season when they fired managers Thomas Tuchel and then Graham Potter.
The hosts started the west London derby on the front foot and winger Noni Madueke almost broke the deadlock in the first half when his curling shot hit the crossbar with 10 minutes on the clock.
But Brentford soaked up everything Chelsea could throw at them with centre forward Nicolas Jackson, back in the starting 11 after an injury, unable to provide much of a threat.
Brentford leap-frogged their neighbours into 10th spot, while Chelsea dropped to 11th ahead of the rest of the weekend's fixtures.
