Former Real Madrid coach Carlo Ancelotti has warned the club that they should not rely on "second chances" in the Champions League.
A Cristiano Ronaldo hat trick steered Madrid through to the last four as they recovered from a 2-0 first leg deficit to Bundesliga side Wolfsburg in the quarterfinals.
And despite the club's long tradition of dramatic comebacks at the Bernabeu, Ancelotti warned that they should not expect to recover every time.
Speaking to Marca, the coach -- who led Madrid to their record 10th Champions League victory (La Decima) -- said what had gone wrong at Wolfsburg should be examined to make sure it did not happen again.
"In the Champions League, you do not get second chances," he added. "If Real Madrid want to repeat another comeback in the semis, it will be very difficult.
"Zinedine Zidane and his players should analyse closely the first leg in Wolfsburg. Another comeback will not be so easy, but they are now just three games from the Undecima (11th European Cup triumph)."
Meanwhile, Barcelona crashed out of the competition to Atletico Madrid, meaning Luis Enrique's team were unable to become the first side to retain the trophy since the 1990 AC Milan team that featured Ancelotti in midfield.
Ancelotti said: "It shows once again how difficult it is to defend the Champions League, even if you have Lionel Messi, Neymar and Luis Suarez.
"Barcelona this season have played 12 games more than their rivals. It seems their [physical] state is not so good, and now they need to fight for La Liga and the Copa del Rey."
He praised Diego Simeone's Atletico side for their impressive organisation as they beat Barca 2-0 on Wednesday night to progress to the last four.
"[Atletico] are a model team for their unity, solidarity of effort and the way they strictly stick to their tactical discipline," he added.