Jose Mourinho's dreams of winning the Champions League with Chelsea suffered an early setback when the Londoners fell to a shock 2-1 home defeat by Swiss champions Basel in their opening Group E match.
Chelsea, beaten in the Premier League by Everton on Saturday, looked short of ideas all night in a stuttering display, though they led 1-0 following a neat finish by Oscar just before halftime.
The Brazilian hit the bar soon after the restart as Chelsea briefly turned on the style but they were never able to take complete control and Mohamed Salah equalised in the 71st minute.
Basel, beaten by Chelsea in the semi-finals of the Europa League last season, grew in confidence and scored again 10 minutes later when captain and centre forward Marco Streller cleverly glanced in a near-post header from a corner.