Sergio Ramos has warned Tottenham that they face a Real Madrid backlash at the Bernabeu on Tuesday.
Sporting Gijon pulled off a shock on Saturday by winning 1-0 in Madrid to inflict Real's first home defeat of the season - and Jose Mourinho's first in nine years including spells with Porto, Chelsea and Inter Milan.
The Real coach has virtually conceded the Spanish title to Barcelona so as defender Ramos stressed before the first leg clash with Spurs, Mourinho's men will now be even determined to claim European glory this season.
"After the defeat against Sporting Gijon it's difficult for us to win La Liga, so the Champions League becomes our obligation," he said.
"It is impossible for us to fail at this stage, in the quarter-finals.
"We ended the match against Sporting very unhappy, but now we have a chance to change the situation. We are focused on Tottenham.
"People say Real are the big favourites in this round, but in the quarter-finals nobody writes off their rivals. Tottenham are tough opponents, but if we don't beat Spurs and go through, it will be a disaster."
Goalkeeper Iker Casillas went further, saying a 10th European crown for Real is a must this season.
"The Liga is very complicated for Real Madrid as the Champions League is our obsession," he said. "Nothing else is sufficient for a club like Real Madrid.
"What happened in the league looks decisive but our big challenge is to progress in Europe. That is so important to us."
Van der Vaart threat
Portuguese star Cristiano Ronaldo is in a race against time to be fit for Real but Casillas insisted: "We can win without Cristiano, no matter how good a player he is."
And Ramos sees Spurs' former Real forward Rafael van der Vaart as the biggest threat to the Spanish giants.
"He demonstrated his quality at Real and now is a key man for Spurs," he added.
"We need to stop Rafael to avoid problems - that is clear. For him this is a special match and he will be up for it."