Chelsea 1-2 Basel
Jose Mourinho's first Champions League tie since returning to Chelsea ended in disappointment as Swiss champions Basel earned a shock win at Stamford Bridge.
Oscar put the home side ahead with an angled first-time shot into the bottom corner at the end of the first half.
Basel's Mohamed Salah levelled when he fired across Blues keeper Petr Cech.
And the home crowd was left stunned as skipper Marco Streller won the Group E opener for the visitors with a late near-post header.
But the Blues, who have won the Champions League and Europa League in the past two seasons, expected a change in fortunes as they returned to European action.
Instead Basel consigned Chelsea to their first Champions League group stage defeat at Stamford Bridge since October 2003.
The shock loss came exactly six years after Mourinho took charge of the final match of his first Blues spell, a 1-1 draw against Norwegian side Rosenborg.
The Portuguese, who returned from Real Madrid in the summer, has played down his youthful squad's chances of winning the Champions League - describing them as "young eggs who need nurturing".
But the team he selected on Wednesday could hardly be described as inexperienced. Including £30m debutant Willian, Chelsea dominated possession in the opening hour, but only had Oscar's strike to show for it.
The home side forced six corners in a dominant first-half display, without being able to force Basel goalkeeper Yann Sommer into a serious save.
Sommer did find himself in trouble when he fumbled an Oscar set-piece under pressure from Gary Cahill and home debutant Samuel Eto'o in his six-yard box, only to be rescued by referee Daniele Orsato who generously awarded a foul against the home players.
Chelsea were finally rewarded just before the interval when Oscar fired a first-time shot into the bottom corner after latching on to a perfectly weighted throughball by Frank Lampard.
The Brazil midfielder also hit the crossbar after the break as Chelsea searched for the security of a second goal. But it did not arrive and Basel, dangerous on the counter-attack throughout, punished their hosts.
Salah, who scored on the same ground when the teams met in last season's Europa League semi-final, had repeatedly tested Chelsea left-back Ashley Cole and it was no surprise that he was the man to level the scores.
Neat build-up from the visitors ended in Streller picking out the unmarked Egypt international, who curled a low left-foot shot beyond Cech.
Mourinho introduced Juan Mata and Demba Ba as the Blues pressed for a late winner, but they only looked more vulnerable as Basel grew in confidence.
The home side scrambled clear after midfielder Fabian Schar burst through the middle, before team-mate Marcelo Diaz had a shot blocked.
But the Swiss side continued to press and grabbed the decisive goal when Streller escaped two Chelsea markers to head home with eight minutes to go.
Basel and German side Schalke, who earned a comfortable 3-0 win against Romanian visitors Steaua Bucharest, top Group E after the opening round of matches.