At a Glance
The London club's hopes of securing a place automatically for the European Cup knockouts were dealt a blow as their Italian opponents overturned a deficit to win in their home stadium.
Each club went into this game with the same records in the initial stage of this year's competition, having secured three victories, tied one, and lost to sit on 10 points.
For large parts there was not much to separate them in this encounter too, even though Chelsea went ahead in the opening 45 minutes when Joao Pedro converted Reece James' delivery.
But The Serie A side always looked a threat going forwards, with former Premier League attacker creating issues for the backline.
James fired wide in the second half but the home team increased the intensity and rightfully levelled when former West Ham United striker Gianluca Scamacca a cross.
The winger hammered a strong effort straight at the keeper but Atalanta got the winner late on when the Chelsea defenders retreated from Charles de Ketelaere, who made no mistake with a powerful strike.
Chelsea attempted to rescue a draw late on but a brilliant save by the Atalanta goalkeeper denied Joao Pedro.
The final score means Chelsea slip out of the automatic qualification places—the positions that qualify automatically for the knockouts—to eleventh with two league phase matches left to play.
Atalanta, meanwhile, climb up to third place, three points ahead of the Blues.
This was a extremely frustrating outcome for Chelsea as even a draw was leaving them well placed to secure the top eight finish they require to bypass the uncertainty of the play-offs.
But they let their concentration slip in their defensive duties with seven minutes left on the clock and Atalanta capitalised completely.
The Italians are in rich vein of form at the moment so this was always going to be a challenging examination for the visitors, but they will be frustrated to be leaving with no points after securing the early advantage.
But the broader context is that this extends Chelsea's concerning run of results in all tournaments.
Since defeating the Spanish giants 3-0 at the end of November they are now without a win in their last four games.
Chelsea boss Maresca enacted five alterations for this game as he persisted with his run of rotating players in and out of the starting lineup.
The rotations here followed after he implemented the identical amount for the loss to Leeds United and six for the draw with Bournemouth prior to the journey to Atalanta.
The coach will see squad rotation as vital as he attempts to handle players' minutes and challenge both at domestically and abroad, but he insisted the key nucleus of players remains the same for crucial fixtures.
"I believe tonight [in] the starting lineup we had eight or nine players who played against Tottenham, FC Barcelona, and Arsenal," he said.
"If you look at the five changes that we made compared to Bournemouth it is a change but it involves the eight or nine players playing Barcelona.
"The players who took to the field tonight are the ones playing almost all the matches."
The Blues' next Champions League fixture sees them welcome the Cypriot side on the coming Wednesday (8pm GMT).
Atalanta, meanwhile, host Athletic Bilbao at the identical kick-off time.
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