The fourth-tier side, whose last competitive match was in March, beat top-flight clubs Cologne and Fortuna Duesseldorf on their way to becoming the first team from that level to reach the last four of the cup.
Saarbruecken coach Lukas Kwasniok had said a place in the July final at Berlin's Olympic Stadium would be tantamount to the "rebirth of Jesus", but there was no miracle on Tuesday.
"We lost, that is bad for our state of mind most of all. It bothered me that we weren't tough enough," said Kwasniok told Sky.
The away side were quickly two ahead thanks to goals from Moussa Diaby and Lucas Alario within the first 20 minutes.
The hosts never recovered from those early blows, and just before the hour mark Leverkusen struck a third, with Demirbay once again the man to lay on the decisive pass for Bellarabi to smash home.