Porto captain Maicon headed a decisive second-half goal to give Porto a 2-1 Champions League win and extend Chelsea's laboured start to the season.
Julen Lopetegui's side were offered encouragement throughout the Group G clash by the defensive frailties that have undermined Chelsea over the past two months and opened the scoring at the Estadio do Dragao through Andre Andre.
Willian – recalled to the starting line-up following his weekend equaliser against Newcastle United, as Jose Mourinho benched Eden Hazard and Nemanja Matic – levelled matters with a brilliant free-kick on the stroke of half-time.
But, as they did at St James' Park on Saturday, Chelsea switched off and Maicon capitalised in the 52nd minute to ensure a miserable return to his old stomping ground for Mourinho.
The margin of victory could have been greater as Danilo headed against the post inside the final 10 minute, while the result moves Porto onto four points alongside Dynamo Kiev – one ahead of Chelsea.
Iker Casillas set a Champions League record by making his 152nd appearance in the competition and the Porto goalkeeper was called into action early on.
Diego Costa, performing at his menacing best in the midst of a domestic suspension, left defenders in his wake down the left and found Cesc Fabregas, only for Casillas to save plunging to his right.
The ex-Real Madrid captain came out on top in another all-Spanish duel in the 14th minute – saving with his feet from Pedro, who charged onto Willian's crisp pass.
Some uncertain Chelsea defending ended with Vincent Aboubakar, who scored both Porto's goals in their opening 2-2 draw at Dynamo Kiev, lashing wildly over.
Full-back Maxi Pereira produced some more wayward shooting for Porto, but the sense of defensive vulnerability surrounding Chelsea was confirmed when they fell behind six minutes before the break.
Yacine Brahimi cut inside from the left past Branislav Ivanovic to draw a brilliant save from Asmir Begovic, who could not keep out Andre's volleyed follow-up.
It was all square from the final, stunning kick of the first-half – William fizzing a wonderful free-kick over the defensive wall and across a statuesque Casillas.
Costa won the foul for the equaliser and combined with Willian to have a 48th-minute attempt deflected wide before Chelsea fell behind again.
Brahimi skipped around Ivanovic again to win a corner and Maicon peeled off the Chelsea goalline to send a clever back header beyond Begovic at the near post from Ruben Neves' delivery.
There was more rough treatment for Ivanovic at the hands of Brahimi, although the Serbian recovered to make a vital block on his tormentor, before Chelsea failed to fully clear and Neves had a deflected attempt headed away from the goalmouth by Kurt Zouma.
Mourinho sent on Hazard in place of John Obi Mikel in the 62nd minute and he was quickly into the action – collecting Pedro's pass and hurdling Maicon's lunging challenge to shoot in to the side netting.
But Chelsea's problems at the other end showed no sign of abating.
Begovic kept out a stinging drive by Giannelli Imbula in the 70th minute and the ball barely left the visiting penalty area during a sustained assault that culminated in Brahimi skipping past Ramires and missing out on the goal he deserved.
Ivanovic passed up a headed set-piece opportunity and Danilo went even closer in the 82nd minute, setting up a frantic period of stoppage time where Porto's defence managed to scramble to safety and deny the tireless Costa at the death.
