Atalanta came from behind to beat Chelsea 2-1 in Bergamo, a late Charles De Ketelaere winner punishing Enzo Maresca's side and denting their hopes of qualifying automatically for the Champions League knockouts.
Both teams arrived with identical league-phase records of three wins, a draw and a defeat, sitting level on 10 points. For long spells there was little to separate them, but Atalanta's persistence and Chelsea's familiar defensive lapses ultimately decided it.
Atalanta start brightly but Chelsea strike first
The hosts began on the front foot, with Ademola Lookman the chief threat. Robert Sanchez denied the former Charlton and Everton attacker once, while Josh Acheampong, on his first Champions League start, earned acclaim from teammates with a point-blank block from the same player.