After a couple of early chances for the hosts Dortmund eventually won comfortably, but Friedrich said that it could have been different had one of the efforts at the start gone in.
"It was eventually [comfortable], but you always need to be focused from start to finish in games like these," he told Bundesliga.com. "Every choice you make can potentially lead to an opportunity for your opponents.
"That happened in the first half when their striker got in behind our defence and was clean through on goal. If that had gone in, it would have been a completely different situation. These games are mentally draining, because you can't afford any slip-ups."
Dortmund's next game is at home to Bayer Leverkusen in the Bundesliga on Saturday.