A late goal from Fernando Torres spared Chelsea considerable embarrassment as they were forced to twice come from a goal down to draw 2-2 away at League One Brentford in the fourth round of the FA Cup.
The cup holders were poor for large swathes of the first meeting of these London sides since 1950, and went behind twice following mistakes from Ross Turnbull as Marcello Trotta thumped home a fine volley after the ball was parried into his path and Harry Forrester converted from the penalty spot following a foul from the Chelsea keeper.
With the hated interim manager Rafa Benitez contemplating the most humiliating result of his brief reign in charge, Oscar first rescued Chelsea with a gorgeous individual effort after 55 minutes, before Torres struck a second equaliser with seven minutes remaining to enjoy a redemptive moment, both for himself and his team.
But the story of the afternoon was a brilliant performance from Brentford, who will now reap the financial rewards of a replay at Stamford Bridge, even if they were unable to topple the cup holders for another cup shock.
Millwall, MK Dons and, most famously, non-league Luton Town had all disposed of Premier League opposition in a fine weekend for the south east's lower league contingent, but seeking to avoid similar humiliation Chelsea were reinforced by the return of captain John Terry, making his first start since suffering a knee injury in November.
Terry certainly looked rusty after 12 minutes when his rushed back pass to Ross Turnbull went wide of the goalkeeper, forcing him to grab it with his hands and concede an indirect free-kick inside the box. Chelsea's massed ranks charged out when the ball was rolled to Forrester and he lifted his effort high and wide.
Outfought in midfield, where their tiny playmakers wilted on a rough, sandy surface against Brentford's more muscular protagonists, Chelsea's first-half display bordered on the inept, with individual errors littering their performance. Gary Cahill's use of the ball was particularly careless, while remarkably a scuffed effort from Oscar was the sum of their efforts in attack.
By contrast, Brentford's superior energy and ambition allowed the League One side to put their Premier League opponents under unexpectedly prolonged pressure. Shaleum Logan and Forrester combined to particularly good effect down the left, with the former teeing up Adam Forshaw for a nice curling effort and the latter having a number of chances for himself.
Indeed, it was from the boot of Forrester that Brentford forced another Chelsea error when scoring the opening goal after 42 minutes. Frank Lampard was culpable in the build-up when losing possession 20 yards from goal and when Turnbull parried Forrester's shot unconvincingly into the path of Trotta, the Italian striker thumped his volley home on the rebound.
After the half-time whistle sounded, Juan Mata was receiving tactical instructions from Benitez even before he reached the tunnel, and the Spaniard was introduced for the second period in the place of Marko Marin, who had disappeared from view in a game that was completely unsuited to his technical, tidy skill-set.
Mata's introduction saw Oscar pushed to the right of the midfield three behind Torres, but it was from a central position where the Brazilian added another goal of extreme technical quality to his slowly growing collection this season.
It was not quite as spectacular as his efforts against Juventus and Shakhtar Donetsk in the Champions League, but Oscar still produced a tidy bit of control on the edge of the box, beat Tony Craig with a fine bit of skill and then found the net with the outside of his boot.
Oscar looked to have given Chelsea the platform to take the game away from inferior opposition, but with Demba Ba curiously remaining on the bench in a game that would have played to his strengths rather than those of Torres, the holders found themselves a goal down once again after 73 minutes.
Tom Adeyemi, just on as a substitute, was sent tumbling by the rash charge of Turnbull to give Brentford a penalty. Forrester's connection wasn't entirely clean but he did just enough to poke the ball under the arm of Turnbull, whose afternoon grew ever worse.
Following a desperate goalmouth scramble, Benitez did make a change but instead of bringing on the prolific Ba, it was Cesar Azpilicueta who came on to replace Ivanovic at right-back. Ba's introduction came three minutes later as he replaced Ryan Bertrand following a glaring headed miss. The striker quickly made an impact.
Torres chested a high ball into the path of Ba and he managed to return the favour with a poked pass as he was brought to ground. The pitch had opened up for Torres on the edge of the box and he curled a lovely finish out of the reach of Simon Moore.
Chelsea could have had a penalty in injury time when Mata's cross struck the arm of Harlee Dean but a defeat would have been unbearably harsh on a Brentford team who in truth merited a place in the fifth round of the cup.
MAN OF THE MATCH
Harry Forrester (Brentford): He assisted the first and scored the second and was a constant threat to Chelsea from the left wing.
BRENTFORD: Moore 7, Hodson 7, Craig 7, Dean 6, Logan 7, Douglas 7, Diagouraga 7, Donaldson 7, Forshaw 8, Forrester 8, Trotta 8. Subs: Adeyemi 7, Barron 6, Saunders 6.
CHELSEA: Turnbull 4, Ivanovic 5, Terry 5, Cahill 4, Cole 6, Ramires 5, Lampard 5, Marin 4, Oscar 7, Bertrand 6, Torres 6. Subs: Mata 7, Azpilicueta 6, Ba 7.