Chelsea suffered another damaging home defeat as goals from Ethan Pinnock and Bryan Mbuemo gave Brentford the bragging rights with a 2-0 win in the west London derby on Saturday.
The big spending Blues have won just one of their last 13 home Premier League games over the past two seasons and this latest setback punctures belief that Mauricio Pochettino was beginning to get Chelsea back on track.
After a difficult start to Pochettino's reign, back-to-back victories over Fulham and Burnley was followed by an impressive performance, even though they blowing a two-goal lead, in a 2-2 draw against Arsenal last weekend.
However, Brentford leapfrogged Chelsea into 10th in the Premier League table, pushing Pochettino's men fall into the bottom half.
Chelsea started brightly, penning the Bees back, but it was a familiar tale for Pochettino as his side lacked the clinical touch to make good approach play count.
The Argentine said on Friday that Christopher Nkunku is nearing a comeback from the knee injury that has prevented the £52 million signing making a competitive appearance yet for his new club.
On this evidence, the French international's return cannot come soon enough as Chelsea's other new striker Nicolas Jackson again failed to fire.
Noni Madueke was unfortunate when his blistering strike from a narrow angle came back off the crossbar after just 10 minutes.
Marc Cucurella should have done much better with a tame effort straight at Mark Flekken after being picked out by the impressive Cole Palmer.
The former Manchester City winger and Raheem Sterling have been the bright sparks as Chelsea's attack has flickered into life in recent weeks.
But Sterling was also guilty of being wasteful when he blazed well off target from close range.
Brentford barely threatened before the break, but gave the hosts a warning at the start of the second period when Yoane Wissa fired into the chest of Robert Sanchez.
Chelsea did not learn their lesson and the defending for the goal will anger Pochettino as much as his forwards' wastefulness.
Mbuemo hung up a cross to the back post and Pinnock took advantage of ball watching by Madueke to power home a header at Sanchez's near post.
After that, it was Brentford who looked the more likely to add to their lead despite Chelsea dominating possession.
Sanchez flew off his line to deny substitute Yehor Yarmolyuk his first Brentford goal before Mbuemo fired inches wide.
Brentford were on the verge of a famous win at Old Trafford earlier this month only for Manchester United to score twice in stoppage time.
However, this time it was Thomas Frank's men who sealed the three points at the death.
Sanchez's decision to come forward for a corner in search of an equaliser backfired as Brentford broke quickly up the other end and Neal Maupay gave Mbuemo the simple task of rolling into an empty net.
Victory means Brentford become the first side to ever win their first three Premier League away games against Chelsea.
AFP