Hannover sit fifth in the table with 10 points from their six matches and have impressed the visiting boss, whose own side are third.
"This is a true top of the table clash," he said. "Two really good sides are playing against each other.
"We are going to have to be in a really good condition to get anything there.
"Hannover did not only impress me this week - had it not been for us, they would be the most amazing success stories in the Bundesliga."
Hannover's 2-1 win over Levante in the Europa League in midweek has provided enough of a warning to Klopp, whose side's last defeat before a recent 3-2 reverse to Hamburg came in the Niedersachsen Stadion last year.
"That was one of the few defeats which actually helped us," admitted Klopp, whose side went on a 31-game unbeaten streak afterwards.
Hannover are on an unbeaten streak of their own right now, not having fallen at home in 21 Bundesliga games.
"We know how strong we are here at home, but statistics are just statistics and they can change quickly," said Hannover's Konstantin Rausch on his club's website.
"We certainly have to show what it is which makes us so strong, and then we can beat Dortmund at home."
Hannover coach Mirko Slomka will be assessing the fitness of his players, who turned a 1-0 deficit against Levante into victory and had to play 80 minutes with only 10 men just 48 hours ago.
Dortmund are certainly without Ilkay Gundogan, who has also had to pull out of the Germany squad after being ruled out for a fortnight.
Meanwhile, Germany defender Mats Hummels is an injury doubt for the defending German champions.