The Spaniard admitted that his team are on the brink of the semi-finals following their first-leg demolition of the Germans, but was also quick to urge caution.
Dortmund famously thrashed Real 4-1 in Germany last season on their way to the Champions League final.
"It's a magnificent result," Isco told Canal+. "We are very happy to get the third goal and not concede. It is a huge step towards the semi-finals.
"But no-one imagined Madrid would lose 4-1 over there. In football you never know."
Dortmund play host to Real for the second leg on April 8.