Griffin Park was rocking as the home fans smelt blood and Brentford put in their best performance of the season to earn their visit to Stamford Bridge in the coming weeks, but despite their lofty position in English football, credit must also be given to Rafa Benitez's side as they showed plenty of character on a day when they could very easily have collapsed and become another of the big guns to be eliminated from this year's competition.
Brentford were nothing short of sensational in the way they harassed and harried a Chelsea team packed full of international stars, although one suspects their chance of progressing to the fifth round have now passed them by.
Bleacher Report takes a look at what we learned from this classic FA Cup encounter in west London.
Marcello Trotta is swamped by his teammates after opening the scoring against Chelsea
With every round of the FA Cup comes the inevitable tales of how the world's oldest domestic cup competition has lost its appeal and is no longer the jewel in English football's crown.
Well, while such sentiments may have been true in recent seasons, it seems the competition is back with a bang in 2013.
In a week that saw League 2 Bradford City become the first club to reach the League Cup Final, the FA Cup was not to be outdone and has restored its sense of being with a host of upsets that once made it the envy of the world.
Brentford came close to adding their name to the list of teams that pulled off shocks this weekend, but their performance was also one of great desire and was a welcome addition to a fine season of cup football.
It's said the FA Cup no longer means what it used to, but try telling that to Brentford fans who saw their team take the European champions all the way and almost come out on top. For that matter, tell the same to Oldham Athletic, Luton Town, Millwall and Leeds United—all teams who knocked out Premier League opposition in the fourth round.
The FA Cup is alive and well.
Rafa Benitez urges his team on against Brentford from the sidelines
The common criticism of Rafa Benitez throughout his managerial career is that he doesn't make good use of his substitutions. Well, his team can be criticized for many things against Brentford, but his substitutions certainly cannot.
Against Swansea City in midweek, many questioned the wisdom in the Spaniard replacing David Luiz and Ashley Cole with fellow defenders as the Blues chased a two-goal deficit against the Swans in the Capital One Cup semifinal.
The Blues crashed out of the competition that day, and with Chelsea up against it once more Sunday when they faced Brentford in the FA Cup, he got things right from the sidelines.
He brought Juan Mata on at the interval to replace Marko Marin, who had struggled throughout the first half. The Spaniard made an instant impact in attack, bringing an edge to Chelsea going forward and they eventually pulled the scores level at 1-1 through Oscar.
And when the Blues fell behind from a Harry Forrester penalty late on, it wasn't long before he introduced Demba Ba to the fray. The Senegalese striker linked up with Fernando Torres for Chelsea to equalize, too, so while the team may have struggled for various reasons on the day, Benitez was influential with his replacements.
It's not going to be often we see a Demba Ba-Fernando Torres double act up front for Chelsea, but Sunday's draw with Brentford outlined that the pair are more than capable of playing together when the Blues are chasing a result.
Such is Chelsea's system, Rafa Benitez would be weakening the team in other areas were he to start the two strikers. However, Torres' well-taken equalizer showed how effective they can be in the latter stages of a game.
Ba had been on the pitch a matter of minutes when Torres cleverly played a one-two with his teammate that resulted in the Spaniard expertly converting from the edge of the box on 83 minutes. It saved Chelsea's blushes on the day, but it more importantly outlined just how effective the strike duo can be.
That goal was Torres' 15th of the season, too. Say what you will about the Spaniard, but that's a fine return for any striker at this stage of the campaign, regardless of his transfer fee.
It's the beauty of cup football—pitting so-called minnows against the giants of the game.
On paper, Chelsea should have walked off the Griffin Park pitch with an easy win to their name, but for all the talent in Rafa Benitez's squad, the Blues were put through the ringer against their west London neighbors and came very close to losing their grip on the FA Cup.
Had Brentford looked to match Chelsea for their style of play and swagger, there's no doubting they would have lost, but instead, they played with a passion and desire that their fans will be talking about for years to come.
Sure, Chelsea are having an unconvincing season by their own standards, but they are European champions for a reason, yet the Bees matched them for the entire 90 minutes.
It was their finest performance and the season and one Brentford will struggle to emulate when these two sides meet at Stamford Bridge for their replay in the coming weeks. That's not to take anything from Uwe Rosler's charges—they were fantastic—but as it tends to do, style will eventually overcome substance.
It used to be that without John Terry and Frank Lampard, Chelsea were a different team. While the Blues have missed the English duo at various stages this season, the days when they were indispensable to the Chelsea cause appear to be over. Juan Mata is now the man who has taken centre stage.
Without Mata, Chelsea are simply not the same team going forward, and minus his threat, teams seem more confident and willing to attack the Blues and cause damage of their own.
We saw that much on Sunday against Brentford. While Chelsea enjoyed plenty of possession in the first half, they lacked a cutting edge that the inclusion of Mata at the interval brought with it.
The Spaniard didn't get an assist or score himself, but his presence gave the Blues more direction in attack and he changed the game in their favour. Without him, Chelsea often struggle and, like Lampard and Terry of old, he is the focal point of this new generation at Stamford Bridge.
Harry Forrester steps up to slot home his penalty against Chelsea
For fans not particularly keen on League 1 football, they would have been given a shock by the performance of Harry Forrester against Chelsea on Sunday.
Well, after Sunday, it's not hard to see why Ajax tried to sign him, and with Forrester available on a free transfer in the summer, teams will be falling over themselves to snap him up.
He was named Man of the Match against Chelsea, and upon reflection, it was an easy decision to make. Forrester was at the heart of everything the Bees created going forward and slotted home his penalty with aplomb as Brentford fans dared to dream of knocking the European champions out of the FA Cup.
It wasn't to be, of course, with Fernando Torres equalizing to make it 2-2, but one thing is for certain, Brentford have a star within their ranks. And whether the club is promoted this season or not, Forrester will not be playing League 1 football next term.