A 95th-minute strike from Rodrigo Muniz saw the Blues lose at Stamford Bridge as they failed to close the gap on Liverpool.
Chelsea dropped more points in the Premier League title race after conceding in the final minute of a 2-1 loss to rivals Fulham on Boxing Day.
Enzo Maresca's Blues have now failed to win in back to back matches against sides from outside the traditional 'big six' and now sit four points behind leaders Liverpool, who boast two games in hand.
Marco Silva set his side up with a back-five formation in the hope of spoiling the hosts' Christmas, but those prayers were quickly rejected when Chelsea took the lead inside 16 minutes through none other than Cole Palmer. Picking up possession midway inside the Fulham half, he weaved his way past two hapless defenders before curling in a strike at the near post from the edge of the box, with the ball rolling between the legs of centre-back Issa Diop on the way in for good measure.
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Fulham went close to an equaliser soon after when Calvin Bassey burst forward from the centre of defence and all the way into the Chelsea box on a run that mimicked that of a tricky winger. He quickly remembered his actual position with a limp finish which was easily saved by Robert Sanchez.
Down the other end, Chelsea ought to have doubled their lead. Enzo Fernandez's delivery from a wide free-kick picked out Marc Cucurella, whose close-range header was tipped away by Bernd Leno and the visitors were able to clear their lines just in time.
On the other side of the interval, Fernandez brought a great save out of Leno himself with a top corner-bound strike from 20 yards, only for the German to claw his effort up and out. Chelsea had the ball in the net from the second delivery of the resulting corner when Palmer's cross was bundled in by Levi Colwill, only for the offside flag to deny them on this occasion.
Fulham grew into the game in the second half and the Blues had Sanchez to thank for staying ahead after he smothered Antonee Robinson's attempt when the USMNT star was given the freedom of the six-yard box.
With little under 10 minutes remaining, Fulham had their leveller. Alex Iwobi forced his way past Pedro Neto and slipped in Robinson, whose cross was knocked back by Timothy Castagne and in for Harry Wilson to prod over the line.
Chelsea quickly rallied to try and find a late winner but were denied by a top stop from Leno when the ball fell to Sancho inside the 18-yard box. With Maresca pushing everyone forward, Fulham hit the Blues on the break for a last-gasp goal of their own. Sasa Lukic broke down the right and found Rodrigo Muniz steaming in to slam home and send the travelling fans crazy.
GOAL rates Chelsea's players from Stamford Bridge...