Jose Mourinho's return to the Champions League with Chelsea started in dismal fashion as FC Basel came from behind to seal a stunning 2-1 win at Stamford Bridge on Wednesday.
Mourinho's first spell at Chelsea came to an acrimonious end exactly six years ago when he was sacked following a lacklustre 1-1 draw against Rosenborg in the Champions League.
And his hopes of erasing that painful memory with a win over Basel in his first match back with Chelsea in Europe's elite club competition since that night were dashed by a remarkable fightback from the Swiss champions.
Brazilian midfielder Oscar had given Chelsea the lead just before half-time, but the Blues paid the price for a sloppy display featuring a host of missed chances as Mohamed Salah equalised after the break and Marco Streller bagged the winner with nine minutes remaining.
"I'm not in shock. The reality is that sometimes you don't win. I'm not happy of course. We have taken a step back in qualification but we have five games left to finish in the top two," Mourinho said in the aftermath of Chelsea's first Champions League group stage home defeat since 2003.
"Basel played very well. We had a lot of the ball but we couldn't create many chances. Emotionally this is not a mature team."
Basel boss Murat Yakin added: "Congratulations to our team. We didn't come here to hide.
"It is great to win, especially after conceding at a difficult moment."
Mourinho has already overseen Chelsea's worst start to a Premier League season for a decade and, just five days after a 1-0 defeat at Everton, he has now suffered a second successive loss and one of the more embarrassing setbacks of his illustrious career.
Blues owner Roman Abramovich was captured shaking his head as the full-time whistle was greeted with boos from a section of the stunned home crowd.
In truth, Chelsea, who have rarely been beaten at home under Mourinho, had never been anywhere near their best and could have few complaints.
When the first real chance finally came after a dour opening half-hour, it was Basel who were on the offensive.
Valentin Stocker slipped a superb pass to Salah, who used his blistering pace to escape Ashley Cole.
But, instead of taking an instant strike at the exposed Petr Cech's goal, the Basel winger opted to cut back inside the penalty area and eventually miscued his shot harmlessly wide.
Despite spluttering for long periods, Mourinho's side took the lead with their first shot on target in the last minute of the first half.
David Luiz brought the ball out of defence and picked out Frank Lampard 20 yards from the goal.
Rather than take a shot, the Chelsea captain unselfishly slipped a perfectly-weighted pass through to Oscar, who drilled a first-time strike low past Yann Sommer.
Mourinho sat unmoved with an impassive look on his face that suggested his disappointment at the preceeding 44 minutes.
The Blues boss was more impressed when, 10 minutes into the second half, Oscar intelligently worked space for a sublime curling shot from the left edge of the penalty area that crashed down off the bar and bounced to safety.
That inventive effort drew applause from Mourinho, and Oscar threatened again moments later as his deflected strike drifted past the far post.
Oscar, a more confident figure than during his underwhelming first season at the club, then showed great vision to play in Hazard, only for the Belgian to shoot over.
But Chelsea's profligacy came back to haunt them when the unheralded Swiss minnows equalised against the run of play in the 71st minute.
A swift interchange of passes carved open the Chelsea defence as Streller gave Salah a sight of goal which he converted in style with a curled finish into the far corner.
Suddenly it was Basel calling the shots and, with Chelsea offering little in response, the visitors stole the win in the 81st minute.
Streller showed more desire and cunning than Gary Cahill as he got in front of his marker to glance a corner past Cech at his near post.