The Bees defended doggedly early on and held the holders at the break after Oscar struck the post from close range.
Here, Sports Mole dissects the 90 minutes that saw Chelsea keep their hopes of retaining the famous trophy alive.
On Target: 10
On Target: 2
Was the result fair?
Rafael Benitez's side undoubtedly deserved to win the match and got their reward for a patient approach with four goals after the interval. Brentford stayed compact and tried to counter but offered little threat in the final third throughout.
A flat first-half effort was quickly forgotten when Mata struck from distance just before the hour mark. Sloppy passing and finishing were a theme early on but eventually the Blues got their reward for consistently putting Brentford under pressure with crosses and movement in the final third.
A spirited display full of passion was undermined by late lapses in concentration. The pace and intensity of the first game against Chelsea were missing in the last 30 minutes as the League One battlers wilted at the Bridge. Rosler's men can be still be proud of their efforts, with Sam Forshaw in particular showing flashes of quality.
Sports Mole's man of the match
Oscar: The Brazilian playmaker continued his fine recent form with a performance full of creativity and desire this afternoon. Oscar capped an effective display by scoring from close range with a clever flick on 68 minutes to make it two goals in as many matches for the 21-year-old.
Brentford felt hard done by in the first half when Marcello Trotta's goal was disallowed to bring play back for an earlier foul on Forshaw. The referee blew up almost instantly when David Luiz dived in but Trotta quickly gained possession and slammed the ball home. A moment to let play develop would have seen the Bees take a shock lead and Uwe Rosler may look back on that as a poor decision.
Neil Swarbrick had a relatively quiet game but could have played advantage to let Brentford register an early goal through Trotta. The Bees striker was furious with the official, who also showed leniency to let Luiz get away with a yellow card after his forearm smash on Jake Reeves.
Chelsea: The Blues are back in Europa League action on Thursday as they defend a one-goal lead in the second leg of their last-32 tie at home to Sparta Prague.
Brentford: The Bees return to their League One promotion push next Saturday at home to play-off rivals Walsall.