The Frenchman, who took over in January from Rafael Benitez, said the Germans may have been going through a rough patch in the Bundesliga in recent weeks but they will be tough opponents at this stage of the competition.
"It will be a complicated match, very difficult," he told reporters. "A team transforms itself at this stage and even if they have problems in the league this is the Champions League. The players know it, the club knows it."
"The are physically very fit. They are quick in attack."
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Wolfsburg have scored one goal in their last three league games, slipping to eighth spot, and could miss out on European action next season.
But Zidane said there was the danger of his side letting their guard down after their 2-1 La Liga victory at Barcelona in the 'Clasico' on Saturday.
"At this stage we (the teams left) are all favourites, every game is a final for us and this game is as well," said Zidane, who lifted the Champions League as a Real player in 2002.
"You can win a game and then relax, let your guard down and you can end up paying the price," he said. "We want to keep our concentration high. My players are concentrated and that is the most important thing for me."
Real, one of two sides along with Barcelona who are still unbeaten in the competition this season, are chasing a record-extending 11th European crown, having lost once in their last 11 European away games.
(Reporting by Karolos Grohmann; editing by Ken Ferris)
