Match Summary
The London club's ambitions of securing a place automatically for the European Cup knockout stages were suffered a setback as Atalanta came from behind to triumph in their home stadium.
Each club went into this game with matching points tallies in the league phase of this year's competition, having won, tied one, and suffered one defeat to sit on ten points.
For extended periods there was very little to separate them in this contest too, even though Chelsea got their noses in front in the opening 45 minutes when Joao Pedro finished Reece James' cross.
But The Serie A side remained a threat going on the attack, with former Charlton and Everton attacker creating issues for the backline.
James fired wide in the second half but the home team upped the intensity and justly drew level when ex-West Ham striker headed in a centering pass.
Alejandro Garnacho drove a powerful shot straight at the goalkeeper but Atalanta secured the decisive strike late on when the Chelsea defence backed off Charles de Ketelaere, who took full advantage with a driven strike.
The visitors attempted to salvage a point late on but a superb stop by the Atalanta goalkeeper denied Pedro.
The result means Chelsea fall out of the automatic qualification places—the spots that qualify automatically for the knockouts—to 11th with two league phase games left to play.
Atalanta, in contrast, leap up to third place, three points ahead of the Blues.
This was a hugely disappointing result for Chelsea as even a draw was leaving them well placed to secure the top-eight finish they need to bypass the uncertainty of the play-offs.
But they let their concentration slip in their defensive duties with seven minutes left on the clock and their opponents took full advantage.
The Italians are in rich vein of form at the moment so this was always going to be a tough test for the visitors, but they will be disappointed to be departing with no points after taking the lead.
But the wider issue is that this extends Chelsea's concerning streak of form in all tournaments.
Since beating Barcelona 3-0 at the end of November they are now winless in their last four games.
Chelsea boss Maresca made five rotations for this game as he continued his strategy of moving players in and out of the starting lineup.
The changes here followed after he implemented the same number for the defeat to Leeds and six for the tie with Bournemouth prior to the journey to Bergamo.
The coach will see squad rotation as vital as he attempts to manage players' workloads and compete both at home and abroad, but he stated the central contingent of players stays the unchanged for key games.
"I think in this match [in] the first XI we had eight or nine players who featured against Tottenham, Barcelona, and Arsenal," he said.
"If you look at the five changes that we made compared to Bournemouth it is a change but it is the eight or nine players playing in that match.
"The ones who played tonight are the ones who feature in almost all the games."
Chelsea's next European fixture sees them welcome the Cypriot side on Wednesday, 21 January (8pm GMT).
Atalanta, meanwhile, host Athletic Club at the identical kick-off time.
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