At a Glance
The London club's hopes of qualifying automatically for the Champions League knockouts were dealt a blow as their Italian opponents fought back to triumph in Bergamo.
The two teams went into 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 10 points.
For extended periods there was very little to distinguish them in this encounter too, although Chelsea took the lead in the opening 45 minutes when the Brazilian finished Reece James' cross.
But The Serie A side always looked a danger going on the attack, with ex- Charlton and Everton attacker creating problems for the defence.
James fired wide in the after the break but the home team increased the intensity and deservedly levelled when former West Ham United striker Gianluca Scamacca a centering pass.
The winger drove a powerful shot straight at the keeper but they scored the winner in the closing stages when the Chelsea defenders backed off Charles de Ketelaere, who made no mistake with a powerful finish.
Chelsea attempted to salvage a draw in stoppage time but a excellent stop by the Atalanta goalkeeper kept out Pedro.
The outcome means Chelsea fall out of the automatic qualification places—the spots that qualify automatically for the next round—to eleventh with two group stage fixtures remaining.
Atalanta, in contrast, climb up to third, three points ahead of the Londoners.
This was a extremely frustrating result for Chelsea as even a draw was keeping them in a strong position to achieve the top eight finish they need to bypass the uncertainty of the play-offs.
But they lost focus in defence with seven minutes remaining and their opponents took full advantage.
The Italians are in rich vein of form at the moment so this was always going to be a difficult examination for the visitors, but they will be disappointed to be leaving with no points after taking the early advantage.
But the bigger picture is that this extends Chelsea's concerning run of results in all competitions.
Since defeating the Spanish giants 3-0 at the end of November they are now without a win in their last four outings.
Chelsea boss Enzo Maresca enacted five alterations for this match as he continued his policy of moving players in and out of the starting 11.
The rotations here followed after he implemented the identical amount for the defeat to Leeds and six for the draw with Bournemouth prior to the journey to Bergamo.
The coach will see rotation as essential as he attempts to manage players' fitness and compete both at domestically and in Europe, but he maintained the key nucleus of players stays the same for key fixtures.
"I believe tonight [in] the first XI we had eight or nine players who played against Tottenham Hotspur, FC Barcelona, and Arsenal," he commented.
"If you see the five changes that we did relative to Bournemouth it is different but it involves the same eight or nine players playing Barcelona.
"The ones who played this evening are the ones who feature in almost all the games."
Chelsea's next European fixture sees them host Pafos on the coming Wednesday (8pm GMT).
The Italian team, meanwhile, are at home to Athletic Bilbao at the identical kick-off time.
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