Match ends, FC Porto 2, Chelsea 1.
Chelsea's poor start to the season continued as they were beaten by Porto in the Champions League.
Andre Andre gave Porto the lead, volleying in after Asmir Begovic pushed away Yacine Brahimi's effort.
Midfielder Willian equalised on the stroke of half-time with a superb free-kick from the edge of the area.
Maicon put the home side ahead again with a flicked header at the front post, before Diego Costa rattled the bar with a curling shot.
Chelsea's Cesc Fabregas and Pedro both forced Spain team-mate Iker Casillas - making a record 152nd appearance in the competition - into saves early on.
But despite their early pressure, and a late penalty claim for a handball by Ivan Marcano, they came away from the Estadio do Dragao with nothing.
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Problems at the back
Chelsea boss Jose Mourinho surprisingly left out last year's PFA player of the year Eden Hazard, who has struggled for form this season.
On-loan striker Radamel Falcao failed to even make the bench against his and Mourinho's former team.
Meanwhile, Brazil midfield duo Willian and Ramires - who both came on and scored in the 2-2 draw against Newcastle at the weekend - were named in the starting side, as was striker Diego Costa, currently serving a domestic suspension.
But it was in defence that the Blues looked to have the biggest problems when the Portuguese league leaders - who won the competition when Mourinho was in charge in 2004 - went ahead six minutes before half-time.
Full-back Branislav Ivanovic's indifferent season continued when he was too easily beaten by Algerian Brahimi in the lead-up to Andre's goal.
Mourinho described his side's first-half performance against the Magpies as "minus one out of 10" and this tie looked to be heading the same away until Willian levelled with a curling free-kick.
Chelsea have conceded the second most goals (14) in the Premier League this season and Jolen Lopetegui's Porto underlined the London club's problems at the back as Maicon lost his marker to reach a near-post corner and head past Begovic. Danilo also hit the post from another set piece.
Mourinho magic wearing off?
Toiling in 15th place in the defence of their Premier League title, eight points behind leaders Manchester United, Mourinho's side have now suffered five defeats in 11 games in all competitions this season.
The Portuguese manager has had disagreements with the media and differences with his coaching staff as his team have stuttered so far domestically - and Mourinho's troubles appeared to have spilled over into Europe.
Mourinho's side easily defeated Maccabi Tel Aviv 4-0 in their opening group game and Chelsea might have taken a point from a hardworking Porto side.
But they saw efforts either hit the woodwork or deflected wide, while Marcano appeared to handle the ball in the area.
Man of the match - Yacine Brahimi
'Two ridiculous moments cost us'
Chelsea manager Jose Mourinho: "We watch dozens and dozens of repetitions of Porto taking corners. We were completely ready for that and in a moment when the game is under control we concede a goal that is ridiculous.
"They hit the post from a similar mistake. Apart from that, the game was balanced. Two ridiculous moments and we were punished."
Porto coach Julen Lopetegui: "The players attitude was amazing, spectacular. It was our best performance of the season. I have to congratulate my team. This three points are totally deserved. It was tough and unfair to go to the dressing room at half-time with a draw, but we reacted well."
The stats you need to know
- Chelsea have lost more games this season (five) than in the whole of last season
- The Blues suffered their first defeat in 10 European matches against Portuguese opposition, with their last defeat also against Porto in December 2004
- Mourinho's side have conceded 18 goals this season - last season, they conceded their 18th goal on 10 December
- Midfielder Willian scored from a direct free-kick for the third game running
- Porto have scored in each of their past 12 Champions League games
Chelsea face a tricky home game against Southampton on Saturday, while they come up against Dynamo Kiev in their third Group F game in the Champions League on 20 October.
- 2Pereira Páez
- 4Pereira Roque
- 5Marcano SierraBooked at 25mins
- 3Martins IndiBooked at 19mins
- 20Brás AndréSubstituted forLayúnat 80'minutes
- 22DaniloBooked at 82mins
- 6NevesSubstituted forEvandroat 78'minutes
- 25ImbulaBooked at 89mins
- 8BrahimiSubstituted forOsvaldoat 87'minutes
- 1da Silva Arruda
- 17JM Corona
- 24CahillBooked at 41mins
- 28AzpilicuetaBooked at 66mins
- 7Santos do NascimentoSubstituted forMaticat 73'minutesBooked at 79mins
- 12MikelSubstituted forE Hazardat 62'minutes
- 17PedroSubstituted forKenedyat 73'minutes
- 19Diego Costa
- 10E Hazard
- Antonio Miguel Mateu Lahoz
- Shots on Target
Second Half ends, FC Porto 2, Chelsea 1.
Attempt blocked. Kenedy (Chelsea) left footed shot from the centre of the box is blocked. Assisted by Diego Costa.
Attempt blocked. Willian (Chelsea) left footed shot from outside the box is blocked.
Corner, Chelsea. Conceded by Marcano.
Giannelli Imbula (FC Porto) is shown the yellow card for a bad foul.
Willian (Chelsea) wins a free kick in the defensive half.
Foul by Giannelli Imbula (FC Porto).
Diego Costa (Chelsea) wins a free kick in the attacking half.
Foul by Evandro Goebel (FC Porto).
Substitution, FC Porto. Pablo Daniel Osvaldo replaces Yacine Brahimi.
Attempt missed. Nemanja Matic (Chelsea) left footed shot from outside the box is high and wide to the left. Assisted by Cesc Fàbregas.
Danilo Pereira (FC Porto) is shown the yellow card for a bad foul.
Eden Hazard (Chelsea) wins a free kick in the attacking half.
Foul by Danilo Pereira (FC Porto).
Danilo Pereira (FC Porto) hits the left post with a header from the centre of the box. Assisted by Miguel Layún with a cross following a corner.
Corner, FC Porto. Conceded by Gary Cahill.
Substitution, FC Porto. Miguel Layún replaces André André.
Nemanja Matic (Chelsea) is shown the yellow card for a bad foul.
Foul by Nemanja Matic (Chelsea).