Harry Maguire was on the scoresheet as Manchester United blew the Premier League's top-four race wide open with a controversial 2-0 victory at misfiring Chelsea.
Maguire was fortunate to escape a red card for kicking Michy Batshuayi in the groin, while Chelsea saw goals for Kurt Zouma and Olivier Giroud chalked off by the Video Assistant Referee (VAR).
England defender Maguire twisted the knife on the sloppy Blues, meeting Bruno Fernandes' corner with a thumping header to confirm a seismic Red Devils win in west London.
Anthony Martial buried United's sole chance of the first half as Ole Gunnar Solskjaer's men punished Chelsea's profligacy to register a first league double over the Blues in 32 years.
Batshuayi missed two glorious first-half chances, the Belgium striker deputising for England hitman Tammy Abraham who missed out with ongoing ankle issues.
Blues boss Frank Lampard has already branded VAR a "passion killer", and two ruled-out goals here will only cement his frustrations with the innovation.
Chelsea could have opened up a nine-point gap on United with victory on Monday night. Instead the Red Devils now trail Lampard's men by just three points.
Tottenham head to Stamford Bridge on Saturday, where a win for Spurs would now allow Jose Mourinho's men to leapfrog Chelsea for the first time this season.
Chelsea's ninth defeat of the season and fifth at home accounts for their struggles to hold onto a top-four berth.
Add this stuttering defeat to the lacklustre home losses to West Ham, Bournemouth and Southampton, and any more missteps could see Chelsea slip out of the Champions League qualification berths.
Chelsea tried and failed to bolster their frontline in January, and another blank in front of goal only underscores the lack of quality and depth available to Lampard.
This was yet another microcosm of Chelsea's season of travails in front of goal, especially before the break.
Willian overcooked a cutback to leave Batshuayi flailing for any kind of connection for the first gilt-edged chance.
Pedro then threaded a delicate through-ball for Mason Mount, and the midfielder's pull back laid it on a plate for Batshuayi, but the struggling striker stubbed his effort woefully wide.
Willian was booked for diving in a poor decision from referee Anthony Taylor when Fernandes had clearly fouled the Blues winger.
And then Taylor rejected any kind of punishment when Maguire kicked out and connected with Batshuayi's groin.
United buried their sole chance of the half, when Aaron Wan-Bissaka skinned Willian and whipped in a smart cross that Martial flicked into the far corner.
Then Batshuayi drilled another shot well wide to close the half.
Chelsea's miserable misfortune continued after the turnaround, as replacement centre-half Zouma saw his potential equaliser chalked off by VAR.
The French defender turned home a corner in style, to show the misfiring Blues forwards how to finish - only for Cesar Azpilicueta's push on Brandon Williams to be ruled a foul.
Chelsea will doubtless lament the fact that Fred pushed Azpilicueta into Williams, leaving the Blues captain with little option but to make contact with the United man.
Fernandes struck the post with a cute United free-kick, but worse was to follow for the Blues.
Just when the hosts expected to pile on the pressure for an equaliser, Portuguese star Fernandes delivered a pinpoint corner and Maguire headed home in style.
Another killer blow was somehow in store for the Blues, as Giroud nodded in from Mount's cross - only for VAR to chalk off a second Chelsea goal, this time for offside.