Levante handed Barcelona a shocking 2-1 defeat to make the Catalans chances of qualifying in the round of 16 clash a bit slim.
The duo of Erick Cabaco and Borja Mayoral scored in the opening 20 minutes to put the host in a comfortable lead at half-time.
Phillipe Coutinho however scored a late penalty to give the Catalans their only goal in the match.
Barca who are defending champions in the Copa del Rey recently lashed Levante three weeks ago by 5-0 in a league match in which Lionel Messi scored a hat-trick but bowed in their first encounter in the league cup.
Barca must be ready to overturn the second-leg to keep their hopes of progressing to the next stage alive.