Shaun Wright-Phillips returned to haunt Chelsea as the Premier League's bottom side QPR pulled off a sensational 1-0 win at Stamford Bridge.
The winger, a first-half substitute for the injured Junior Hoilett, smashed a low strike into the far corner on 78 minutes to stun the Blues after man-of-the-match Adel Taarabt laid off a brilliant ball for the former Chelsea player.
QPR stay bottom on goal difference and remain five points from safety, but Rs fans will hope that such an unexpected and uplifting derby win will give Harry Redknapp's men a fighting chance of survival.
The recent goodwill of Chelsea fans towards Rafael Benitez, meanwhile, could be at an end after a demoralising defeat. The Spaniard, who received a lousy reception when appointed to replace Roberto Di Matteo, had turned things around recently.
However his decision to rotate his squad against the Premier League's bottom side, with Marko Marin making his first league start for Chelsea as Juan Mata and Eden Hazard were left on the bench, could return to haunt him.
He lived to regret that in a lacklustre first half. Either of the two rested players could have been key to unlocking a resolute Rangers defence as Marin struggled to impress and Frank Lampard was unable to get forward past the banks of red shirts.
Indeed Marin was lucky to feature beyond the fourth minute. Having lost control of the ball as he came inside from the right, he lunged in on the shin of Stephane Mbia with his studs up. Referee Lee Mason took an exceptionally sympathetic view of the incident in only issuing a yellow card.
Ryan Nelsen and Clint Hill were giants in dealing with Chelsea's crosses, but QPR midfield general Shaun Derry was fortunate to not be booked at least once despite grabbing first Marin then Oscar round the neck as they attacked in separate incidents.
Harry Redknapp had brought Derry back in following the 3-0 home loss to Liverpool and also recalled Fabio, Esteban Granero and Hoilett, with Djibril Cisse and Wright-Phillips dropped to the bench and Armand Traore and Samba Diakite left out of the matchday squad altogether.
Hoilett limped out after a quarter of an hour with Wright-Phillips replacing him, but the former Chelsea star could not make an impact on the break as his side lacked confidence on the ball in the opening 45 minutes.
Taarabt worked hard in an unfamiliar position alone up front and was sometimes let down by the movement of his team-mates when looking to prise open the home defence.
The closest Chelsea came to breaking the deadlock was just before the half-time whistle when Oscar tuned to shoot and forced a bizarre save from Julio Cesar with his legs as he dived the wrong way then shortly after Victor Moses avoided a challenge before sending a rising drive just over the bar.
The home fans were crying out for changes at the break but Benitez kept things the same. It seemed that his patience may be rewarded as Chelsea piled on the pressure immediately.
Three minutes in, sublime skill from Marin on the left allowed him to pick out Moses in the six-yard box, but he somehow contrived to miss as he lunged for the ball.
On 52 minutes Branislav Ivanovic grazed the bar with a header and a minute later Fernando Torres, who cut a clownish figure in attack at times, pounced on a blocked David Luiz shot only to see Cesar pull off a brilliant point-blank save at his near post.
QPR began to believe in themselves around the hour-mark with an unexpected flurry of attacks. The most promising of them saw Jamie Mackie played in by a disguised Taarabt pass with Gary Cahill recovering to execute a last-ditch tackle.
Hazard was finally introduced for Marin just before that and a clever drag-back from him on 66 minutes afforded Moses space to send a deflected shot just wide of the far post.
The resulting corner was scrappily dealt with and Moses played in Lampard, who finished into the far bottom corner from close range. However the linesman's flag ruled it out and replays appeared to show him to be level.
The captain took it in good grace, but his side were left to rue the decision even more when QPR took a shock lead. Wright-Phillips won a corner off Cesar Azpilicueta and when it was cleared, Taarabt laid off a wonderful one-touch ball reminiscent of Pele for Carlos Alberto for Wright-Phillips to crash home from 25 yards.
The former Stamford Bridge star, who was given a friendly reception from the home fans when introduced, refused to celebrate.
A West London derby which had featured many bad tackles finally saw its second yellow card five minutes from time as Hill chopped down Ivanovic dead centre 20 yards out. Rather than allowing Mata - on for Moses with a quarter of an hour left - the chance to curl one in, Luiz blasted straight into the 10-man QPR wall.
Ivanovic headed over as the match entered injury-time and when Luiz blazed over in the third of three minutes added on, the home fans began to stream for the exits.
The title dream barely ignited under Benitez is surely now firmly extinguished, while QPR - who took just a second clean sheet in 27 away matches - could see the benefit from this match beyond these three crucial points.
MAN OF THE MATCH
Adel Taarabt (QPR) - Worked his socks off alone up front and his lay-off for Shaun Wright-Phillips's winner was Pele-esque.
Chelsea: Turnbull 6; Azpilicueta 5, Ivanovic 6, Cahill 7, Bertrand 6; Luiz 5, Lampard 6, Moses 6, Oscar 6, Marin 6; Torres 4. Subs: Ramires 5, Mata 6, Hazard 6.
QPR: Julio Cesar 7; Onuoha 6, Nelsen 7, Hill 7, Fabio 6; Mbia 6, Derry 6, Granero 6; Taarabt 8, Mackie 6, Hoilett 6. Subs: Park 6, Wright-Phillips 7, Dyer 6.