Key Moments
Chelsea's chances of securing a place automatically for the Champions League knockouts were hit hard as Atalanta fought back to triumph in Bergamo.
Each club entered this fixture with matching records in the league phase of this year's competition, having secured three victories, tied one, and suffered one defeat to sit on 10 points.
For extended periods there was little to choose between them in this encounter too, although Chelsea went ahead in the opening 45 minutes when Joao Pedro turned in the captain's cross.
But Atalanta remained a danger going forwards, with former Charlton and Everton attacker Ademola Lookman problems for the backline.
James shot wide in the after the break but Atalanta increased the intensity and rightfully levelled when former West Ham United striker headed in a centering pass.
Alejandro Garnacho drove a strong effort straight at the keeper but Atalanta got the winning goal in the closing stages when the Chelsea defenders backed off Charles de Ketelaere, who took full advantage with a low shot.
Chelsea tried to salvage a draw in stoppage time but a excellent stop by the Atalanta goalkeeper kept out Joao Pedro.
The outcome means Chelsea drop out of the top eight—the spots that qualify automatically for the knockouts—to 11th with two league phase matches remaining.
The Bergamo club, in contrast, jump up to third place, three points ahead of the Londoners.
This was a hugely disappointing outcome for Chelsea as even a point was leaving them in a strong position to secure the top-eight position they need to avoid the uncertainty of the play-offs.
But they lacked concentration in their defensive duties with seven minutes left on the clock and Atalanta capitalised completely.
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 annoyed to be departing with nothing after taking the early advantage.
But the wider issue is that this extends Chelsea's troubling streak of form in all tournaments.
Since defeating the Spanish giants 3-0 at the close of November they are now without a win in their last four matches.
Manager Enzo Maresca enacted five rotations for this game as he persisted with his strategy of rotating players in and out of the starting lineup.
The rotations here followed after he implemented the same number for the loss to Leeds and six for the draw with AFC Bournemouth prior to the journey to Atalanta.
Maresca will see rotation as essential as he attempts to handle players' workloads and compete both at home and abroad, but he maintained the core group of players remains the same for important fixtures.
"I believe in this match [in] the first XI we had eight or nine players who featured against Tottenham, FC Barcelona, and the Gunners," he said.
"If you look at the five alterations that we did compared to Bournemouth it may be a change but it is the same eight or nine players who played in that match.
"The players who took to the field this evening are the ones who feature in almost all the matches."
Chelsea's next European fixture sees them host Pafos on Wednesday, 21 January (8pm GMT).
Atalanta, on the other hand, are at home to Athletic Club at the same time.
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