Chelsea's Champions League aspirations suffered a significant setback on Tuesday evening as Enzo Maresca's side fell to a 2-1 defeat against Atalanta in Bergamo.
The Blues arrived in Italy having failed to win in their previous three fixtures, with their last victory coming in the impressive triumph over Barcelona a fortnight ago.
That barren run has now extended to four matches without success.
The loss sees Chelsea slip from the automatic qualification places down to 11th in the league phase standings, leaving their hopes of securing a top-eight finish hanging by a thread.
Atalanta turned on the style after half-time as Chelsea faltered in humiliating fashion
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Atalanta, meanwhile, surge up to third position with 13 points, three clear of the London club.
Joao Pedro opened the scoring for the visitors on 25 minutes, latching onto a precise low delivery from captain Reece James and guiding the ball through goalkeeper Marco Carnesecchi's legs at the near post.
The strike was initially chalked off for offside against James, but a lengthy VAR examination determined the England defender had timed his run perfectly.
It marked Pedro's maiden goal in the Champions League.
Chelsea had taken the lead against the run of play, with Atalanta having enjoyed the better of the opening exchanges.
Chelsea looked ponderous in possession and were punished for it as they let their one-goal lead slip
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Ademola Lookman proved particularly troublesome for the visitors, with young defender Josh Acheampong producing a crucial block to deny the former Everton forward from close range.
The second period told a different story entirely, with the Serie A side returning from the interval with renewed purpose.
Chelsea squandered an opportunity to extend their advantage early on, James firing wide from a promising position inside the area.
Lookman thought he had drawn Atalanta level shortly after, only for the offside flag to cut short celebrations.
The equaliser arrived on 55 minutes when De Ketelaere worked space down the right flank and delivered an inviting cross that Scamacca, left inexplicably unmarked in the six-yard box, nodded past Robert Sanchez.
With seven minutes remaining, De Ketelaere completed the turnaround, driving past Marc Cucurella and unleashing a fierce strike that Sanchez could only parry into the roof of the net.
Chelsea's path to automatic qualification now appears fraught with difficulty, with the Blues almost certainly requiring victories in both of their remaining league phase fixtures.
Atalanta moved into an automatic qualification spot in the Champions League while Chelsea are now facing a mighty uphill struggle
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One of those encounters sees them travel to face Antonio Conte's Napoli in January, a daunting prospect against the Serie A champions.
The defeat marks the first time since 2014 that Chelsea have lost a Champions League match having opened the scoring.
Pedro had a golden opportunity to salvage a point with virtually the final kick of the contest, but Carnesecchi produced a superb save to preserve Atalanta's advantage.
Substitute Alejandro Garnacho also tested the Italian goalkeeper, while James blazed over during stoppage time as the Blues' late surge proved fruitless.