Holders Real Madrid will need to maintain their intensity for the whole of next week's return leg if they are to get past Atletico Madrid into the Champions League semi-finals, coach Carlo Ancelotti said on Tuesday.
Real dominated their city rivals in the first half of Tuesday's quarter-final, first leg at Atletico's Calderon stadium but were unable to convert a host of chances.
Spanish champions Atletico then fought back after halftime, with both sides having chances to break the deadlock before the repeat of last season's final ended 0-0.
Atletico goalkeeper Jan Oblak was in spectacular form and Ancelotti singled out Diego Simeone's side as "one of the best defensive teams in the world" at his post-match news conference.
"Above all in the first half we deserved more than a 0-0 but that's the way it happened," the Italian told reporters.
"The first half was fantastic and we played with a lot of personality," he added.
"We did some great work going forward with some excellent movement but then we didn't finish off the moves well enough."
"The second half was different, Atletico pressuring more and we found it harder to control possession."
"The result isn't so good but we have confidence due to the game we played above all in the first half and for the return."
"What we have to do is try to repeat the way we played for the first 45 minutes for 90."
Simeone said the tie remained "very open" and praised his players for the way they turned the match around following the break after being overrun in the opening period.
"We have a very intelligent group of players who knew how to react to the way things went in the first half," Simeone told reporters.
"They (Real) are playing the second leg in front of their own fans and they will make a huge effort so we have to try and keep our heads," added the Argentine coach.
Each side will be missing a player through suspension for the return on April 22 after Atletico midfielder Mario Suarez and Real left back Marcelo picked up bookings.
Ancelotti and Simeone said they had not seen an incident in the second half in which Real defender Dani Carvajal appeared to throw a punch at Atletico forward Mario Mandzukic.
Mandzukic also suffered a bang to the head in an aerial clash with Sergio Ramos that infuriated the bloodied Croatia striker, who remonstrated for several minutes after the clash.