Match Summary
The London club's hopes of securing a place automatically for the European Cup knockouts were dealt a blow as Atalanta came from behind to triumph in their home stadium.
The two teams went into this game with matching records in the initial stage of this year's tournament, having secured three victories, drawn, and lost to sit on ten points.
For much of the game there was not much to separate them in this contest too, although Chelsea got their noses in front in the opening 45 minutes when the Brazilian turned in the captain's cross.
But The Serie A side consistently appeared a threat going on the attack, with ex- Charlton and Everton attacker creating issues for the defence.
James fired wide in the after the break but Atalanta raised the intensity and justly equalised when former West Ham United striker Gianluca Scamacca a cross.
Alejandro Garnacho hammered a powerful shot straight at the keeper but Atalanta scored the winner in the closing stages when the Blues defenders retreated from Charles de Ketelaere, who made no mistake with a driven strike.
The visitors tried to salvage a point in stoppage time but a brilliant stop by the Atalanta goalkeeper denied Pedro.
The final score means Chelsea slip out of the top eight—the spots that qualify automatically for the knockouts—to 11th with two league phase games left to play.
Atalanta, meanwhile, jump up to third, three points ahead of the Blues.
This was a hugely frustrating outcome for Chelsea as even a point was keeping them in a strong position to secure the top eight position they need to avoid the uncertainty of the qualifying play-off round.
But they let their concentration slip in their defensive duties with seven minutes left on the clock and their opponents took full advantage.
The Italian side are in good form at the moment so this was always going to be a tough test for the Blues, but they will be annoyed to be departing with no points after taking the early advantage.
But the bigger picture is that this continues Chelsea's troubling run of form in all competitions.
Since beating Barcelona 3-0 at the close of November they are now without a win in their last four games.
Manager Enzo Maresca made five alterations for this match as he continued his policy of rotating players in and out of the starting lineup.
The rotations here followed after he made the same number for the loss to Leeds United and six for the tie with Bournemouth prior to the journey to Bergamo.
The coach will see squad rotation as necessary as he tries to handle players' workloads and challenge both at domestically and abroad, but he stated the core group of players remains the same for important games.
"I believe tonight [in] the first XI we had eight or nine players who played against Tottenham Hotspur, Barcelona, and Arsenal," he commented.
"If you look at the five changes that we did relative to Bournemouth it may be different but it involves the eight or nine players who played Barcelona.
"The players who took to the field tonight are the individuals playing almost all the matches."
The Blues' next Champions League fixture sees them host the Cypriot side on Wednesday, 21 January (20:00 GMT).
Atalanta, on the other hand, host Athletic Club at the identical kick-off time.
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