Substitute Fabio Carvalho struck right at the death to deny Chelsea all three points as Brentford showed grit to clinch a share of the derby spoils.
It was the home side who fashioned the better chances during the first half and Kevin Schade broke the deadlock in style with an ice-cool finish after he was sent racing clear courtesy of a majestic pass from Jordan Henderson.
Enzo Maresca rung the changes at half-time in a bid to jolt Chelsea into life and the introduction of Marc Cucurella, Reece James and Tyrique George did improve the Blues, the latter of whom went close to drawing them level with an angled snapshot which needed save from Caoimhin Kelleher.
Cole Palmer was the next member of the cavalry called upon and it took the Blues' talisman less than five minutes to find the net after making his return from injury. Both sides traded jabs before Moises Caicedo unleashed a thunderbolt beyond Kelleher in the 85th minute to complete the Chelsea turnaround.
But Carvalho - a 90th minute substitute - was in the right place at the time time to divert a long throw into the net to spoil the Chelsea party. Here are our player ratings from The Gtech...
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ChelseaRobert Sanchez 6 - Had little chance with the first Brentford goal and made some good stops before their late leveller.
Wesley Fofana 5 - Didn't look fully convincing and was no surprise to see Reece James replace him at half-time.
Tosin Adarabioyo 6 - Was kept busy by Igor Thiago and Schade before Chelsea upped the ante in the second half.
Trevoh Chalobah 7 - Made a superb last-ditch intervention to prevent Igor Thiago from getting through on goal early in the second half.
Jorell Hato 5 - Struggled to adapt to have the same impact as Marc Cucurella when drifting infield from the left. Taken off at the break.
Enzo Fernandez 6 - Has grown into an influential figure in the engine room. Like others, improved after the break.
Moises Caicedo 8 - Was one of the better players in a difficult first half before stepping up to thunder home what it looked like would be the winning goal.
Pedro Neto 7 - Direct and made things happen. Would have had more than one assist if his attacking team-mates were in better form.
Facundo Buonanotte 5 - A debut to forget for a player whose move to Stamford Bridge took many by surprise. Taken off at the break.
Jamie Gittens 5 - Another who was ineffective in the first half. Replaced by Cole Palmer just as it seemed he was finding his feet.
Joao Pedro 6 - Popped up here there and everywhere and was more involved after the break.
SubsMarc Cucurella 6 - A key cog in Enzo Maresca's team. Chelsea looked far more balanced and threatening with him and Reece James on.
Reece James 6 - Chelsea looked far more comfortable with their captain at right-back.
Tyrique George 6 - Forced an early save from Caoimhin Kelleher after being introduced at half-time. Injected some life into Chelsea's attack before fading.
Cole Palmer 7 - Chelsea's talisman stepped off the bench when they needed him most. Plundered a classy equaliser less than five minutes after coming on.
Alejandro Garnacho 6 - Was only given 10 minutes to make an impact on debut. Teed up Moises Caicedo but failed to track Fabio Carvalho for Brentford's leveller.
Caoimhin Kelleher - 7
Michael Kayode - 6
Sepp van den Berg - 7
Nathan Collins - 6
Ethan Pinnock - 6
Keane Lewis-Potter - 6
Mathias Jensen - 6
Jordan Henderson - 7
Yehor Yarmolyuk - 6
Igor Thiago - 7
Kevin Schade - 7
SubsRico Henry - 5
Dango Ouattara - 5
Aaron Hickey - 5
Fabio Carvalho - 6
Kristoffer Ajer - 5
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