At a Glance
Chelsea's chances of qualifying automatically for the Champions League knockouts were hit hard as Atalanta overturned a deficit to win in Bergamo.
The two teams went into this fixture with matching records in the initial stage of this season's competition, having won, tied one, and lost to sit on 10 points.
For much of the game there was very little to separate them in this contest too, although Chelsea went ahead in the first half when Joao Pedro finished the captain's delivery.
But Atalanta remained a threat going forward, with ex- Charlton and Everton attacker creating issues for the backline.
James fired wide in the after the break but the home team raised the pressure and justly drew level when former West Ham United striker Gianluca Scamacca a cross.
Alejandro Garnacho hammered a strong effort straight at the goalkeeper but they got the winning goal in the closing stages when the Blues defenders retreated from Charles de Ketelaere, who took full advantage with a driven finish.
The visitors tried to salvage a point late on but a excellent stop by Marco Carnesecchi denied Joao Pedro.
The final score means Chelsea drop out of the automatic qualification places—the spots that progress automatically for the knockouts—to eleventh with two league phase matches remaining.
Atalanta, meanwhile, leap up to third place, three points ahead of the Londoners.
This was a deeply frustrating outcome for Chelsea as even a draw was leaving them well placed to secure the top eight position they require to bypass the uncertainty of the qualifying play-off round.
But they lacked concentration in their defensive duties with seven minutes remaining and their opponents capitalised completely.
The Italians 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 frustrated to be leaving with nothing after taking the lead.
But the bigger picture is that this extends Chelsea's troubling run of results in all tournaments.
Since beating Barcelona 3-0 at the end of November they are now without a win in their last four games.
Chelsea boss Maresca made five alterations for this game as he continued his policy of moving players in and out of the starting 11.
The changes here followed after he implemented the same number for the loss to Leeds United and six for the tie with Bournemouth prior to the journey to Atalanta.
The coach will view squad rotation as vital as he attempts to handle players' workloads and compete both at domestically and in Europe, but he stated the key nucleus of players stays the unchanged for important fixtures.
"I think in this match [in] the first XI we had eight or nine players who played against Tottenham, Barcelona, and Arsenal," he stated.
"If you look at the five alterations that we did compared to the previous game it is a change but it involves the eight or nine players who played in that match.
"The ones who played this evening are the individuals playing almost all the matches."
Chelsea's next Champions League fixture sees them host the Cypriot side on the coming Wednesday (8pm GMT).
The Italian team, on the other hand, host Athletic Club at the identical kick-off time.
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