Celta Vigo held Barcelona to a 1-1 draw in the first leg of the round of 16 in the 2018 Copa del Rey on Thursday.
Jose Arnaiz opened the scoring for the Blaugrana after 15 minutes, but the hosts equalised through Pione Sisto midway through the first half at the Estadio Municipal de Balaidos.
Celta were impressive as they restricted Barca's usual dominance, and they have a puncher's chance of success when they travel to Catalonia for the return leg.
Barca fielded a rotated team, with long-term absentee Ousmane Dembele not selected as a starter as he makes his recovery in the squad.
Gerard Pique and Thomas Vermaelen anchored the side, with Dembele readied on the bench after returning from injury.
Celta fielded a 4-3-3 as they attempted to play on the front foot, and the hosts were effective as they battled their opponents with intent.
Iago Aspas troubled Barca in the opening stages, but the attacker could not find the net as he threatened to grab the lead.
The player regretted his miss on 15 minutes as the La Liga leaders took the lead through Arnaiz's finish.
Andre Gomes was the creator, setting up Arnaiz after crossing from the left wing for the opener.
Celta maintained their early confidence as they quickly attempted to fight back, and they were rewarded 16 minutes later from the foot of Sisto.
The Denmark international slammed the ball home from a tight angle after Aspas caused havoc in the Barca penalty area.
It was all the hosts deserved, and Barca were content to enter half-time with the score tied.
Blaugrana manager Ernesto Valverde was frustrated in the opening exchanges of the second half, with Sergio Busquets hitting the woodwork after Denis Suarez spurned a good opportunity.
The visitors started to dominate possession in typical fashion, but Celta remained dangerous on the counter-attack as they desired a winner.
Dembele returned to action with 20 minutes remaining, prompting applause as the Frenchman resumed his career after a significant spell on the sidelines.
However, the former Borussia Dortmund attacker was rusty, and Celta appeared motivated to grab a late goal in order to control the tie.
Barca were unsettled in the closing 10 minutes of the game, but the Sky Blues failed to take the chance their endeavours deserved.
The visitors will be satisfied with their work at Balaidos, but the tie hangs in the balance ahead of the second leg in a week's time.