Up for a corner deep in stoppage time, the Mainz custodian's shot ricocheted off defender Andreas Beck and then Hoffenheim's Yunus Malli to earn the hosts a point and end a four-game losing streak.
"We got this point in the end with the will and courage of the team in the second half," Muller told Sky television.
"I was lucky to get the header and for it to go in in the end."
Mainz fought back from conceding two early goals, showing excellent morale in view of their four previous defeats.
"We never let our heads drop," said coach Thomas Tuchel. "It's certainly an achievement to maintain a positive body language.
"Games are never lost and they are never won until the final whistle."
Hoffenheim coach Markus Gisdol was "extremely frustrated" to miss out on all three points.
"We worked very hard for 75 minutes to get this win, and then we threw it all away in quarter of an hour," he said.