The BVB coach praised Frankfurt for the way it clawed itself back into the game temporarily and was delighted that set piece practice paid off in the cup win.
BVB dominated the majority of the first half and finished with aplomb in the DFB-Pokal quarterfinal at the Commerzbank Arena, although the hosts threatened to take the lead early in the second half.
Klopp admits that his side was outclassed after half-time but was thrilled with Pierre-Emerick Aubameyang's headed goal from a Henrik Mkhitaryan corner after 83 minutes, vindicating recent set piece practice.
"The first half was clearly ours and we played really well for 70 minutes," he told reporters. "Unfortunately, in the first 20 minutes after the break, we had our rhythm broken and no clear way to win the game.
"After that there were many interruptions to play that helped return the momentum back our way. Frankfurt also did very well themselves and had some opportunities.
"We have trained a lot for set pieces, now it has worked out for us two games in a row. This is very important. We really wanted to be in the DFB-Pokal semifinals, and it's something I think my team can win."
Dortmund faces Frankfurt again on Saturday, this time in the Bundesliga and at Signal Iduna Park.