Arsenal checked Chelsea's top-four hopes after Jorginho's dreadful error gifted Mikel Arteta's side a 1-0 win at Stamford Bridge.
The midfielder's ill-conceived backpass caused havoc in the 16th minute, with Emile Smith Rowe the beneficiary, ensuring the Gunners doubled up on their 3-1 Boxing Day win over Chelsea.
Thomas Tuchel made seven changes to the side that beat Premier League champions and forthcoming Champions League final foes Manchester City last Saturday, seemingly with an eye on this weekend's FA Cup final against Leicester City.
Chelsea, who remain fourth having missed out on the chance to climb above their Wembley opponents, almost levelled when Kurt Zouma and Olivier Giroud both hit the crossbar in the final minute, as the battle for Champions League qualification took another twist.
Kai Havertz caught Pablo Mari napping and surged through on goal in the 10th minute, only to lift his shot wastefully over.
That mistake paled next to the wild backpass Jorginho sent fizzing towards his own net. A scampering Kepa Arrizabalaga did very well to push the ball off his goal line but Pierre-Emerick Aubameyang got to the rebound quickest and set up Smith Rowe to finish.
Mason Mount led the Chelsea response, first having a shot blocked from Christian Pulisic's cutback before linking well with Havertz to draw a sharp stop from Bernd Leno.
Arsenal wing-back Bukayo Saka erred in trying shepherd the ball out in the 36th minute, his England colleague Ben Chilwell drilling at Leno before Mohamed Elneny cleared behind with Mount lurking.
Tuchel made an attacking half-time substitution, with Callum Hudson-Odoi coming on for Billy Gilmour and Mount reverting to a deeper midfield role.
Chelsea remained on the front foot and thought they were level with an hour played, although Pulisic was offside when he converted Havertz's flick on from Mount's left-wing corner.
Elneny was tackled by Mount dangerously near to the Arsenal area but Giroud – freshly on for Havertz against his former club – sliced wildly off target.
A stretching Pulisic shot too close to Leno with five minutes to play and the Arsenal goalkeeper touched Zouma's header onto his bar before Giroud went similarly close with the follow-up.
What does it mean?
Liverpool play twice before Chelsea are next in league action, by which point they could be a point behind the Blues, while West Ham can also cut the gap. Next Tuesday's league encounter with Leicester could yet prove every bit as important to Chelsea as what lies in wait at the national stadium.
Chelsea will look to add a trophy to their phenomenal revival under Tuchel in Saturday's FA Cup final. Arsenal are next in action seven days from now at Crystal Palace.