A youthful Atletico Madrid side came back from a goal down to beat third-tier Reus 2-1 on Tuesday to edge towards the last 16 of the Copa del Rey.
Lucas Hernandez's defensive error allowed Fran to fire the Catalans into a shock lead on the half hour mark.
However, 10-time winners Atletico responded immediately through Luciano Vietto's headed equaliser before Saul Niguez flicked home the winner 27 minutes from time.
The two sides meet again in the second leg on December 17.
"The tie is still tight and it isn't a comfortable result," said Atletico boss Diego Simeone.
"I value the ability to come back from 1-0 down.
"Once we start making better decisions in the box it will lift the level of the team a lot."
Simeone made seven changes to the side that beat Espanyol on Saturday to remain second in La Liga.
And that lack of continuity showed early on against a Reus side that had only been beaten once previously this season on their way to leading their regional division in the third tier.
Vietto should have opened the scoring when he fired straight at Edgar Badia with a clear sight of goal after seven minutes.
However, the opener arrived at the other end as Hernandez slipped under pressure from Fran and the playmaker confidently slotted past Miguel Angel Moya.
Vietto made amends for his earlier miss with a fine header into the bottom corner to level six minutes later.
Atletico had far more control after the break, but needed a set-piece to break Reus down as Saul's clever flicked header from Koke's corner looped into the far corner.
In the night's other tie, second division Leganes shocked Granada 2-1.
Holders Barcelona are in action on Wednesday when they host Villanovense in the second leg of their fourth round tie at the Camp Nou after a 0-0 draw in Extremadura five weeks ago.
Real Madrid also open their Cup campaign on Wednesday away to Cadiz.