Key Moments
The London club's chances of securing a place automatically for the European Cup knockout stages were dealt a blow as Atalanta came from behind to claim victory in their home stadium.
The two teams entered this fixture with matching records in the initial stage of this year's competition, having won, drawn, and lost to be on ten points.
For large parts there was little to separate them in this contest too, although Chelsea got their noses in front in the first half when Joao Pedro converted Reece James' cross.
But Atalanta remained a danger going forward, with ex- Charlton and Everton attacker creating problems for the backline.
James fired wide in the second half but the home team increased the pressure and justly drew level when ex-West Ham striker nodded home a cross.
Alejandro Garnacho hammered a strong effort straight at the keeper but Atalanta secured the decisive strike late on when the Blues defenders backed off Charles de Ketelaere, who took full advantage with a driven strike.
Chelsea tried to salvage a draw in stoppage time but a brilliant save by Marco Carnesecchi denied Pedro.
The result means Chelsea drop out of the top eight—the spots that qualify automatically for the next round—to 11th with two league phase games left to play.
Atalanta, meanwhile, leap up to third, three points ahead of the Londoners.
This was a hugely disappointing outcome for Chelsea as even a draw was keeping them in a strong position to achieve the top eight finish they need to avoid the uncertainty of the play-offs.
But they lost focus in their defensive duties with seven minutes left on the clock and their opponents seized the moment.
The Italian side are in good form at the moment so this was always going to be a challenging test for the Blues, but they will be frustrated to be leaving with nothing after securing the lead.
But the wider issue is that this continues Chelsea's concerning streak of results in all competitions.
Since defeating Barcelona 3-0 at the close of November they are now without a win in their last four outings.
Manager Enzo Enzo Maresca made five changes for this game as he continued his run of moving players in and out of the starting 11.
The changes here followed after he made the same number for the loss to Leeds and six for the draw with Bournemouth prior to the journey to Atalanta.
Maresca will see squad rotation as vital as he attempts to handle players' fitness and compete both at home and abroad, but he stated the central contingent of players stays the same for important fixtures.
"I believe tonight [in] the first XI we had eight or nine players who played against Tottenham, Barcelona, and the Gunners," he commented.
"If you see the five changes that we did relative to the previous game it may be different but it involves the same eight or nine players playing Barcelona.
"The ones who took to the field this evening are the individuals who feature in almost all the games."
The Blues' next European fixture sees them welcome the Cypriot side on the coming Wednesday (20:00 GMT).
The Italian team, meanwhile, host Athletic Club at the same time.
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