Captain Iker Casillas said Real Madrid were given a Champions League wake-up call as they lost 2-0 to last season's beaten finalists Borussia Dortmund in their quarter-final second leg match but still went through to the last four 3-2 on aggregate on Tuesday.
Germany winger Marco Reus scored both Borussia's goals to spark hopes of the hosts reaching the semi-finals.
However, Real held on but patently missed their star performer Cristiano Ronaldo, who was on the bench because he had not recovered fully from the knock to the knee he took in the first leg.
"Maybe it's good for us to receive a wake-up call like that from time to time," said Casillas.
"We committed errors at the back that cost us and we cannot commit in the semi-finals.
"It's better that it came tonight and not in the upcoming games we have.
"After half-time we had to be mentally strong.
"When the scoreboard says 2-0 after half an hour, you know it will be difficult, especially after missing a penalty, but sometimes you have to grind it out."
Real squandered the chance of an early lead after Angel Di Maria had his penalty saved by Dortmund captain Roman Weidenfeller.
"We're happy to be in the semi-final, that was our main goal, but the style of our win certainly left something to be desired," said Real coach Carlo Ancelotti.
"Things could have gone differently if we had converted our early penalty.
"That was the turning point, it made the opposition more bold and my players felt some fear.
"We then made some mistakes in defence and we practically gave away two goals.
"Perhaps we used our heads better in the second-half and defended better, so we managed it at the end."
Dortmund coach Juergen Klopp delivered on his pre-match promise that his side would bow out in style.
"This game is one for the archives, a video must be made and all teams who lose the first leg 3-0 must be shown it to say, 'it can still happen'," said Klopp.
"I am proud of the image the boys have given of Borussia Dortmund."
Dortmund could easily have finished with the 4-0 result they needed to take them through as midfielder Henrikh Mkhitaryan hit the post in the second-half with the goal at his mercy, then fired a shot straight at Casillas.
"Neither of his chances were easy," said Klopp.
"It happens in football, we missed other chances, Kevin (Grosskreutz) had one and we missed five or six chances in Madrid.
"With a goal at Real and one extra tonight, we'd have been through, but it would be great to judge the match as a whole and not only on the two missed chances."
Real's Sergio Ramos said Ancelotti's half-time team talk had made the difference.
"That is what the manager is there for and perhaps if we had gone out with the same attitude in the second-half, we wouldn't be talking about playing in the semi-finals," he said.
"We need to be content because it is not easy to be in the semis four years in a row."