Joao Pedro's first-ever Champions League goal put Chelsea ahead in the first half in Bergamo.

But the Blues lost their momentum after the interval and former West Ham striker Gianluca Scamacca grabbed the equaliser before De Ketelaere, who provided the assist for Atalanta's leveller, capped his influential display with the late winner.

Chelsea found it impossible to subdue Belgium forward De Ketelaere, who has six goals and seven assists in his last 12 Champions League appearances.

Since their memorable 3-0 win over Barcelona in the Champions League on November 25, Chelsea have gone four matches without a victory in all competitions.

Enzo Maresca's 11th-placed side have 10 points from six league phase games and their bid for a top eight finish -- which guarantees automatic qualification for the last 16 -- is now on a knife-edge.

Chelsea's last two matches are in January at home against Pafos and away to Napoli.

Third-placed Atalanta have enjoyed wins over Club Brugge, Marseille, Eintracht Frankfurt and Chelsea this season as they chase a last 16 berth.

Maresca made five changes from Saturday's 0-0 draw at Bournemouth in the Premier League, with Cole Palmer rested after his return last weekend from a two-month injury absence.

There was no place for Chelsea's emerging teenage star Estevao, who has three Champions League goals this season and starred against Barcelona.

- Lethargic Chelsea -

Maresca's reshuffled line-up nearly fell behind inside five minutes when Ademola Lookman weaved into the area for a cross that deflected off Chelsea goalkeeper Robert Sanchez before it was hacked to safety.

Chelsea struggled to find their rhythm early on and when Enzo Fernandez worked a sight of goal, the midfielder blazed wildly over.

Lookman was a thorn in Chelsea's side and it took a last-ditch block from Josh Acheampong to deny the former Everton winger, with the rebound curled just over by Marten de Roon.

Acheampong roared and clenched his fists in celebration of his goal-saving intervention.

And Chelsea made the most of that escape to take the lead in the 25th minute.

When Joao Pedro deftly slotted in Reece James' cross from four yards, the goal was initially disallowed for offside before a VAR check ruled in favour of the Chelsea forward.

It was only Joao Pedro's third goal for Chelsea in his last 16 games in all competitions.

James should have doubled Chelsea's lead when he smashed just wide from the edge of the area early in the second half.

It was a crucial miss and Atalanta made James pay as they drew level in the 55th minute.

De Ketelaere's cross to the far post caught Chelsea's defenders flat-footed and Scamacca made a perfectly timed run to head home from close range.

Scamacca almost punished Chelsea's sloppy marking again moments later with a glancing header that Sanchez saved at full stretch.

Maresca responded to Chelsea's lethargy by sending on Alejandro Garnacho, who forced an immediate save from Marco Carnesecchi.

But Chelsea were creaking at the back and De Ketelaere snatched the winner in the 83rd minute, advancing unchecked to the edge of the area for a powerful strike that deflected in off Marc Cucurella.