Key Moments
Chelsea's ambitions of qualifying automatically for the Champions League knockout stages were hit hard as Atalanta came from behind to triumph in Bergamo.
The two teams went into this fixture with matching points tallies in the initial stage of this season's tournament, having won, drawn, and lost to be on ten points.
For extended periods there was little to separate them in this encounter too, even though Chelsea got their noses in front in the opening 45 minutes when Joao Pedro converted the captain's delivery.
But Atalanta always looked a danger going on the attack, with former Charlton and Everton attacker creating problems for the backline.
Reece James shot wide in the second half but Atalanta upped the intensity and rightfully levelled when former West Ham United striker nodded home a centering pass.
The winger hammered a powerful shot straight at the goalkeeper but Atalanta got the winner in the closing stages when the Blues defenders retreated from Charles de Ketelaere, who made no mistake with a low shot.
Chelsea attempted to salvage a draw late on but a brilliant stop by Marco Carnesecchi kept out Joao Pedro.
The result means Chelsea drop out of the top eight—the positions that progress automatically for the knockouts—to eleventh with two league phase matches left to play.
The Bergamo club, meanwhile, leap up to third, three points ahead of the Londoners.
This was a deeply disappointing outcome for Chelsea as even a draw was keeping them well placed to secure the top eight finish they need to bypass the lottery of the play-offs.
But they lost focus in their defensive duties with seven minutes left on the clock and Atalanta seized the moment.
The Italian side are in rich vein of form at the moment so this was always going to be a difficult test for the visitors, but they will be disappointed to be departing with nothing after securing the early advantage.
But the wider issue is that this extends Chelsea's concerning sequence 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 made five alterations for this game as he persisted with his run of moving players in and out of the starting lineup.
The changes here came after he made the same number for the defeat to Leeds United and six for the draw with AFC Bournemouth prior to the journey to Atalanta.
The coach will see squad rotation as necessary as he tries to manage players' workloads and challenge both at home and in Europe, but he stated the core group of players stays the same for important fixtures.
"I think in this match [in] the starting lineup we had eight or nine players who played against Tottenham Hotspur, Barcelona, and Arsenal," he stated.
"If you look at the five alterations that we made relative to the previous game it is a change but it is the eight or nine players playing Barcelona.
"The ones who took to the field tonight are the individuals playing almost all the games."
The Blues' next European fixture sees them welcome the Cypriot side on the coming Wednesday (20:00 GMT).
Atalanta, on the other hand, are at home to Athletic Bilbao at the same time.
A digital strategist with over a decade of experience in tech innovation, focusing on helping businesses adapt to emerging technologies.