Memories of the 5-4 thriller at Ciutat de Valencia last season that cost Ernesto Valverde's men an unbeaten LaLiga campaign came flooding back during an end-to-end first half in which Levante thrived.
Jeison Murillo and Chumi, afforded opportunities amid a raft of Barca changes, floundered at the heart of defence and the hosts took full advantage, storming to an early two-goal advantage thanks to Erick Cabaco and Borja Mayoral.
The absence of Lionel Messi and Luis Suarez hampered the holders' hopes of extracting a result although they do have a goal to take back to Camp Nou on January 17 thanks to Philippe Coutinho's potentially pivotal 85th-minute penalty.
Barca's experimental defensive partnership of debutant Murillo, on loan from Valencia, and teenager Chumi came unstuck in the fourth minute as Cabaco rose unchallenged to head in Ruben Rochina's free-kick.
Emmanuel Boateng squandered two good chances to double the advantage but made amends in the 18th minute, prodding through a pass for Real Madridloanee Mayoral to dispatch into the bottom far corner.
Opportunities were flowing at both ends with Ousmane Dembele and Malcom both failing to beat Aitor Fernandez from promising positions.
Jasper Cillessen routinely came to Barca's rescue in the opening 45 minutes and the Netherlands goalkeeper did so again just beyond the hour, rushing off his line to foil substitute Jose Luis Morales in a one-on-one.
Valverde introduced Arsenal-linked Denis Suarez off the bench and it was the midfielder's mazy dribble that invited Coke into a clumsy challenge, allowing Coutinho to give Barca a much-needed foothold ahead of the return encounter.
Stars will return for second leg
The return leg of this tie is one of three matches Barca must negotiate within the space of eight days, but this disappointing result leaves Valverde with little scope to rest the likes of Messi, Suarez and Gerard Pique next time around.
That is, of course, unless the Blaugrana boss decides a fifth successive Copa del Rey triumph is not among the club's pressing priorities.
Mayoral makes Madrid proud
He took his goal with aplomb and Mayoral was an influential factor throughout Levante's proactive performance.
As if to provide a service to parent club Madrid, the Spanish striker saved his best for the LaLiga leaders, complementing the first-half strike with two key passes, one of which should have resulted in a goal for team-mate Morales.
