Fabio Carvalho tapped in a 93rd-minute equaliser from a long throw to earn Brentford a 2-2 draw after Moises Caicedo had smashed Chelsea ahead late on.
Cole Palmer scored within five minutes of coming off the bench to cancel out Kevin Schade's first-half opener and launch a second-half turnaround.
Palmer was introduced in the 56th minute on his return from a groin injury after Enzo Maresca had made a triple substitute at half-time following a poor first-half performance that saw them have just one shot on target.
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Reece James and Marc Cucurella came on for Wesley Fofana and Jorrel Hato, who both started at full-back, while Tyrique George replaced Facundo Buonanotte as Chelsea piled on the pressure before Caicedo blasted them in front with a shot from the edge of the area with only five minutes to play.
However, Brentford refused to give in and Michael Kayode's long throw dropped for the unmarked Carvalho at the back post to snatch a point.