Match Summary
The London club's ambitions of securing a place automatically for the Champions League knockouts were suffered a setback as their Italian opponents overturned a deficit to win in their home stadium.
The two teams entered this match with matching records in the league phase of this year's competition, having secured three victories, tied one, and lost to have accrued 10 points.
For extended periods there was not much to separate them in this contest too, although Chelsea got their noses in front in the first half when Joao Pedro finished Reece James' delivery.
But The Serie A side consistently appeared a threat going forwards, with former Premier League attacker causing issues for the defence.
Reece James shot wide in the after the break but the home team upped the pressure and deservedly levelled when former West Ham United striker Gianluca Scamacca a centering pass.
The winger hammered a strong effort straight at the goalkeeper but they secured the winner in the closing stages when the Chelsea defenders backed off the midfielder, who took full advantage with a powerful strike.
Chelsea tried to salvage a point in stoppage time but a superb stop by Marco Carnesecchi denied Pedro.
The result means Chelsea fall out of the automatic qualification places—the positions that progress automatically for the next round—to 11th with two group stage fixtures remaining.
Atalanta, in contrast, climb up to third place, three points ahead of the Blues.
This was a extremely disappointing outcome for Chelsea as even a point was keeping them well placed to secure the top-eight finish they need to avoid the lottery of the qualifying play-off round.
But they let their concentration slip in defence with seven minutes remaining and Atalanta took full advantage.
The Italians are in fine fettle at the moment so this was always going to be a tough examination for the visitors, but they will be frustrated to be departing with no points after taking the lead.
But the wider issue is that this extends Chelsea's troubling run of results in all competitions.
Since beating Barcelona 3-0 at the close of November they are now winless in their last four outings.
Manager Enzo Enzo Maresca enacted five changes for this match as he continued his run of moving players in and out of the starting lineup.
The changes here followed after he made the same number for the defeat to Leeds and six for the tie with AFC Bournemouth prior to the journey to Bergamo.
Maresca will see squad rotation as necessary as he tries to handle players' minutes and compete both at home and in Europe, but he stated the core group of players remains the unchanged for important fixtures.
"I believe tonight [in] the first XI we had eight or nine players who played against Tottenham Hotspur, Barcelona, and Arsenal," he said.
"If you look at the five changes that we made compared to Bournemouth it may be a change but it involves the same eight or nine players who played Barcelona.
"The players who played tonight are the ones who feature in almost all the games."
Chelsea's next European game sees them host the Cypriot side on Wednesday, 21 January (8pm GMT).
Atalanta, meanwhile, are at home to Athletic Club at the same time.
Lena Hoffmann is a seasoned journalist with a passion for uncovering stories that matter, specializing in German current affairs and digital media trends.
