Key Moments
Chelsea's hopes of qualifying automatically for the European Cup knockout stages were dealt a blow as their Italian opponents fought back to claim victory in Bergamo.
Both sides entered this fixture with the same points tallies in the initial stage of this year's competition, having won, tied one, and suffered one defeat to have accrued ten points.
For large parts there was very little to choose between them in this contest too, although Chelsea went ahead in the first half when the Brazilian finished the captain's delivery.
But The Serie A side consistently appeared a danger going forwards, with former Premier League attacker causing problems for the backline.
Reece James fired wide in the after the break but the home team increased the pressure and deservedly levelled when former West Ham United striker headed in a cross.
Alejandro Garnacho drove a powerful shot straight at the keeper but Atalanta secured the winner in the closing stages when the Blues defence retreated from Charles de Ketelaere, who seized the opportunity with a low shot.
Chelsea tried to rescue a point in stoppage time but a excellent stop by Marco Carnesecchi denied Joao Pedro.
The outcome means Chelsea fall out of the automatic qualification places—the spots that qualify automatically for the next round—to 11th with two group stage matches remaining.
Atalanta, meanwhile, jump up to third place, three points clear of the Londoners.
This was a hugely disappointing outcome for Chelsea as even a draw was leaving them well placed to secure the top eight position they require to bypass the lottery of the qualifying play-off round.
But they let their concentration slip in their defensive duties with seven minutes left on the clock and Atalanta capitalised completely.
The Italian side are in rich vein of form at the moment so this was always going to be a difficult examination for the Blues, but they will be disappointed to be leaving with no points after taking the lead.
But the bigger picture is that this extends Chelsea's concerning streak of results in all tournaments.
Since defeating the Spanish giants 3-0 at the close of November they are now without a win in their last four games.
Manager Enzo Enzo Maresca enacted five alterations for this match as he continued his policy of rotating players in and out of the starting 11.
The changes here came after he implemented the identical amount for the defeat to Leeds United and six for the draw with Bournemouth prior to the trip to Atalanta.
The coach will view rotation as essential as he tries to handle players' minutes and challenge both at home and abroad, but he insisted the core group of players remains the unchanged for crucial fixtures.
"I believe in this match [in] the first XI we had eight or nine players who featured against Tottenham, FC Barcelona, and Arsenal," he commented.
"If you look at the five changes that we did compared to Bournemouth it may be a change but it involves the same eight or nine players who played Barcelona.
"The players who took to the field this evening are the ones playing almost all the matches."
Chelsea's next Champions League game sees them host the Cypriot side on Wednesday, 21 January (8pm GMT).
Atalanta, meanwhile, are at home to Athletic Bilbao at the identical kick-off time.
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