Luis Suarez scored two goals to guide Barcelona to a 2-1 win against Atletico Madrid after Fernando Torres put the away side ahead but was then dramatically sent off.
Torres fired Atletico ahead midway through the first half, but was then shown a second yellow card following a needless foul on Sergio Busquets.
Barcelona, who had seen their long unbeaten record ended following a late defeat at home by 10-man Real Madrid in El Clasico on Saturday, rallied in the second half.
Suarez was perhaps fortunate not to be sent off for kicking out at Juanfran, with the incident not spotted by the officials.
But the former Liverpool striker netted a brace to put Luis Enrique’s side in charge of the tie ahead of the second leg at Vicente Calderon on April 13.
It had been a positive start by the home side as Lionel Messi sent a left-foot volley wide before Neymar headed over a floated cross from the right by full-back Dani Alves.
Diego Simeone’s team, who are six points behind the La Liga leaders after trashing Real Betis 5-1, looked dangerous on the break as Antoine Griezmann fired a low shot just wide from around 22 yards out.
Atletico then went ahead on 24 minutes when Koke played a pass between the Barcelona defenders to find Torres in space at the edge of the penalty area, with the Spain forward showing great composure to slot the ball between the legs of goalkeeper Marc-Andre ter Stegen.
Torres then went into the notebook for a foul on Neymar, before setting up Griezmann, whose angled shot was touched behind by Ter Stegen.
On 35 minutes, Atletico were reduced to 10 men when Torres was sent off after German referee Felix Brych showed him a second yellow card for going through the back of Busquets on the half-way line with plenty of team-mates covering.
Barcelona, though, could also have found themselves without Suarez, but the officials failed to spot a kick out by the Uruguay striker at Juanfran in the Atletico penalty area.
Following the restart, the home side took control as Atletico were pressed back into the own half.
Messi’s overhead kick dropped just wide, before Neymar’s rising shot from 18 yards rattled the crossbar and Atletico keeper Jan Oblak saved again from the Argentinian.
The pressure eventually told as Suarez equalised on 63 minutes, bundling the ball in from close range.
There was, however, nothing scruffy about the forward’s second goal of the night, which came on 74 minutes as he headed in Alves’ cross.
It was now all about damage limitation for the visitors, who were perhaps fortunate not to see defender Lucas Hernandez also shown a red card for a cynical foul to stop Neymar’s break, and was shown only a yellow.