Match Summary
The London club's hopes of securing a place automatically for the Champions League knockouts were dealt a blow as Atalanta fought back to claim victory in their home stadium.
Each club entered this match with matching points tallies in the initial stage of this year's tournament, having secured three victories, tied one, and lost to have accrued 10 points.
For large parts there was very little to separate them in this contest too, although Chelsea got their noses in front in the opening 45 minutes when the Brazilian finished Reece James' cross.
But Atalanta always looked a danger going forward, with former Charlton and Everton attacker causing problems for the defence.
Reece James shot wide in the second half but the home team raised the pressure and rightfully equalised when former West Ham United striker Gianluca Scamacca a centering pass.
The winger hammered a strong effort straight at the goalkeeper but they got the decisive strike in the closing stages when the Blues defence retreated from the midfielder, who seized the opportunity with a low strike.
Chelsea tried to rescue a draw late on but a superb save by the Atalanta goalkeeper kept out Joao Pedro.
The result means Chelsea drop out of the automatic qualification places—the spots that qualify automatically for the next round—to 11th with two group stage matches left to play.
The Bergamo club, meanwhile, leap up to third place, three points ahead of the Londoners.
This was a deeply disappointing result for Chelsea as even a draw was leaving them well placed to achieve the top eight position they need to bypass the lottery of the play-offs.
But they let their concentration slip in their defensive duties with seven minutes remaining and Atalanta seized the moment.
The Italians are in fine fettle at the moment so this was always going to be a difficult examination for the visitors, but they will be disappointed to be departing with no points after securing the lead.
But the wider issue is that this continues Chelsea's troubling sequence of results in all competitions.
Since beating Barcelona 3-0 at the close of November they are now winless in their last four matches.
Manager Enzo Enzo Maresca enacted five changes for this game as he continued his run of rotating players in and out of the starting 11.
The rotations here followed after he implemented the same number for the defeat to Leeds and six for the draw with AFC Bournemouth prior to the journey to Bergamo.
Maresca will view rotation as necessary as he tries to handle players' minutes and compete both at domestically and in Europe, but he stated the core group of players stays the unchanged for important fixtures.
"I think tonight [in] the first XI we had eight or nine players who played against Tottenham, FC Barcelona, and Arsenal," he said.
"If you look at the five changes that we made compared to the previous game it may be a change but it is the eight or nine players playing in that match.
"The players who played this evening are the ones who feature in almost all the matches."
Chelsea's next Champions League fixture sees them host Pafos on the coming Wednesday (8pm GMT).
Atalanta, on the other hand, are at home to Athletic Bilbao at the identical kick-off time.
