Key Moments
The London club's chances of securing a place automatically for the European Cup knockouts were suffered a setback as Atalanta fought back to triumph in their home stadium.
Both sides entered this match with identical points tallies in the league phase of this season's competition, having won, drawn, and lost to have accrued 10 points.
For extended periods there was not much to separate them in this encounter too, even though Chelsea took the lead in the first half when the Brazilian turned in Reece James' delivery.
But Atalanta remained a danger going forwards, with ex- Charlton and Everton attacker Ademola Lookman issues for the backline.
James shot wide in the second half but Atalanta raised the intensity and rightfully levelled when former West Ham United striker nodded home a cross.
The winger hammered a strong effort straight at the keeper but they secured the decisive strike late on when the Chelsea defenders backed off Charles de Ketelaere, who took full advantage with a driven finish.
Chelsea attempted to salvage a draw late on but a excellent stop by the Atalanta goalkeeper kept out Joao Pedro.
The final score means Chelsea drop out of the top eight—the positions that qualify automatically for the next round—to eleventh with two league phase games remaining.
The Bergamo club, in contrast, climb up to third, three points clear of the Blues.
This was a hugely disappointing outcome for Chelsea as even a draw was keeping them well placed to achieve the top-eight finish they require to bypass the uncertainty of the play-offs.
But they lacked concentration in defence with seven minutes left on the clock and Atalanta seized the moment.
The Italians are in good form at the moment so this was always going to be a challenging examination for the Blues, but they will be frustrated to be departing with nothing after securing the lead.
But the broader context is that this extends Chelsea's worrying sequence of form in all competitions.
Since beating the Spanish giants 3-0 at the end of November they are now winless in their last four outings.
Chelsea boss Maresca enacted five alterations for this match as he continued his run of moving players in and out of the first team.
The rotations here followed after he implemented the same number for the defeat to Leeds United and six for the draw with AFC Bournemouth prior to the journey to Bergamo.
Maresca will view rotation as essential as he attempts to handle players' workloads and challenge both at domestically and abroad, but he maintained the core group of players remains the same for important games.
"I think tonight [in] the first XI we had eight or nine players who played against Tottenham, FC Barcelona, and the Gunners," he commented.
"If you see the five alterations that we did relative to Bournemouth it is a change but it involves the same eight or nine players who played in that match.
"The players who played this evening are the individuals who feature in almost all the matches."
Chelsea's next Champions League fixture sees them welcome the Cypriot side on Wednesday, 21 January (8pm GMT).
Atalanta, meanwhile, host Athletic Club at the identical kick-off time.
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