Key Moments
Chelsea's chances of qualifying automatically for the European Cup knockouts were hit hard as their Italian opponents overturned a deficit to claim victory in their home stadium.
Each club entered this fixture with identical records in the initial stage of this year's competition, having secured three victories, tied one, and lost to have accrued ten points.
For much of the game there was little to choose between them in this contest too, even though Chelsea got their noses in front in the first half when Joao Pedro converted the captain's delivery.
But Atalanta always looked a danger going forwards, with former Premier League attacker creating issues for the defence.
James fired wide in the after the break but the home team increased the pressure and deservedly drew level when former West Ham United striker Gianluca Scamacca a cross.
The winger hammered a powerful shot straight at the keeper but Atalanta got the decisive strike late on when the Chelsea defenders backed off the midfielder, who seized the opportunity with a low finish.
Chelsea attempted to salvage a draw late on but a superb save by the Atalanta goalkeeper denied Pedro.
The result means Chelsea drop out of the top eight—the positions that progress automatically for the next round—to 11th with two group stage fixtures left to play.
The Bergamo club, in contrast, climb up to third place, three points clear of the Londoners.
This was a hugely frustrating result for Chelsea as even a point was leaving them well placed to secure the top eight position they require to bypass the lottery of the play-offs.
But they lost focus in their defensive duties with seven minutes remaining and their opponents capitalised completely.
The Italian side are in rich vein of form at the moment so this was always going to be a challenging examination for the visitors, but they will be frustrated to be leaving with no points after securing the lead.
But the wider issue is that this continues Chelsea's concerning sequence of results in all tournaments.
Since beating the Spanish giants 3-0 at the close of November they are now without a win in their last four games.
Chelsea boss Maresca enacted five rotations for this game as he continued his policy of rotating players in and out of the starting lineup.
The changes here came after he implemented the same number for the loss to Leeds and six for the tie with Bournemouth prior to the trip to Atalanta.
The coach will see squad rotation as essential as he attempts to handle players' minutes and challenge both at domestically and abroad, but he insisted the core group of players remains the unchanged for crucial games.
"I believe in this match [in] the starting lineup we had eight or nine players who played against Tottenham, FC Barcelona, and the Gunners," he commented.
"If you see the five changes that we did relative to Bournemouth it is different but it is the eight or nine players who played in that match.
"The ones who took to the field this evening are the individuals playing almost all the games."
The Blues' next Champions League fixture sees them welcome Pafos on Wednesday, 21 January (8pm GMT).
Atalanta, on the other hand, are at home to Athletic Club at the same time.
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