At a Glance
Chelsea's hopes of qualifying automatically for the European Cup knockouts were suffered a setback as their Italian opponents fought back to claim victory in their home stadium.
The two teams entered this match with identical points tallies in the initial stage of this season's competition, having won, drawn, and suffered one defeat to sit on ten points.
For large parts there was not much to separate them in this encounter too, even though Chelsea went ahead in the opening 45 minutes when Joao Pedro turned in Reece James' cross.
But The Serie A side remained a danger going forwards, with ex- Charlton and Everton attacker causing problems for the defence.
James shot wide in the after the break but the home team upped the intensity and justly equalised when former West Ham United striker nodded home a centering pass.
The winger drove a strong effort straight at the goalkeeper but Atalanta got the winning goal in the closing stages when the Blues defenders backed off the midfielder, who made no mistake with a driven shot.
The visitors attempted to rescue a point in stoppage time but a superb stop by Marco Carnesecchi denied Joao Pedro.
The result means Chelsea drop out of the automatic qualification places—the positions that qualify automatically for the next round—to 11th with two group stage games left to play.
Atalanta, meanwhile, jump up to third, three points clear of the Londoners.
This was a deeply disappointing outcome for Chelsea as even a point was leaving them well placed to secure the top eight position they need to avoid the lottery of the play-offs.
But they lacked concentration in their defensive duties with seven minutes left on the clock and their opponents capitalised completely.
The Italian side are in fine fettle at the moment so this was always going to be a challenging examination for the visitors, but they will be annoyed to be leaving with nothing after securing the lead.
But the bigger picture is that this continues Chelsea's troubling run of form in all tournaments.
Since beating Barcelona 3-0 at the close of November they are now winless in their last four outings.
Chelsea boss Enzo Maresca enacted five changes for this match as he persisted with his strategy of rotating players in and out of the first team.
The changes here followed after he made the same number for the defeat to Leeds United and six for the draw with Bournemouth prior to the journey to Bergamo.
Maresca will view squad rotation as necessary as he tries to handle players' minutes and challenge both at home and abroad, but he stated the core group of players remains the same for important fixtures.
"I believe in this match [in] the first XI we had eight or nine players who played against Tottenham Hotspur, Barcelona, and Arsenal," he said.
"If you see the five changes that we did relative to Bournemouth it is a change but it is the same eight or nine players playing Barcelona.
"The players who played tonight are the ones playing almost all the games."
Chelsea's next Champions League fixture sees them host Pafos on Wednesday, 21 January (20:00 GMT).
The Italian team, on the other hand, host Athletic Bilbao at the same time.
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