Mourinho guided his team into the last four with an aggregate success over APOEL Nicosia on Wednesday night and then virtually wrote off the chances of meeting his former club in next month's final in Munich.
However, Guardiola will not be taking Roberto Di Matteo's rejuvenated Blues lightly in the two-legged tie later this month.
Speaking at his press conference ahead of tomorrow [Saturday] night's La Liga game at Real Zaragoza, the 41-year-old said: "I'm thankful for the confidence he has in us, but all four teams in the semi-final have a chance.
"It is a team of great experience, that has fantastic players all over the pitch. It is one of the great teams in Europe.
"It is a team with history in recent years in English football. Competing in the Champions League, they have always been there and have shown themselves capable. We will have to produce good performances to reach the final.
"It is clear that in the semi-finals of the Champions League there is no rival that you can think will be easy."
Madrid set up a semi-final against Bayern Munich by completing an 8-2 aggregate victory over APOEL at the Bernabeu in midweek, and the Portuguese told Telemadrid after the match that - should his side advance - he did not expect to find Chelsea waiting in the final.
Asked why he had said that Barcelona would progress, Mourinho said: "Because they are very good. Barcelona aren't the favourites, they are super-favourites."
He added: "Let me be honest, I don't think the final will be a Real Madrid-Chelsea final. It could be Bayern v Barcelona - I just don't think it will be Real Madrid vs Chelsea and we know why."