Heidenreich spent about six months feuding with The Undertaker, and he came away from that time with a very high opinion of "The Phenom."
"He's the ultimate professional, you know?" Heidenreich said during a recent interview with Hannibal TV. "If you can't go in there and wrestle with him and do well, there's something wrong with you. Because he's the best. And the best guys take a new guy who has talent and the right attitutde and they can mold you, in ring. It's on-the-job training in the ring, in front of 10,000 (people)."
The most high-profile match between Heidenreich and Undertaker happened at the 2005 Royal Rumble, when they met in a casket match.
"I actually had a deep-seeded inner fear for caskets. It comes out of something horrific that happened to me but it's something that I drew off of that made it real. My brother was murdered when I was in high school and that's where my fear of caskets really came from," Heidenreich said, adding, "I think that's what made it so believable because it came from somewhere real."
Heidenreich did note his major regret was that he never faced The Undertaker at WrestleMania. Instead, it was Randy Orton who took on The Undertaker and "The Streak" at WWE's annual showcase event in 2005.
"I wanted to work the Undertaker. That'd have been the culmination of almost a whole year," Heidenreich said, adding "I was disappointed, needless to say, but that's their choice."
Heidenreich also shared a story of a match with The Undertaker at a house show in Italy, when the top rope broke shortly after the match began.
"The match is gone, in your mind, of what you're going to do. I was frozen," Heidenreich recalled. "He said, 'Kick my ass, beat the hell out of me, it's easy now.' That was Defcon 50-whatever, to start the match nervous and have the rope break. We modified everything and had the match."
Heidenreich credited his training in Ohio Valley Wrestling, the WWE developmental territory at the time, for learning that ability to improvise.
"Thank God I was trained in OVW where you learn how to work; you don't have to call everything out. [Undertaker] told me, 'When you're upside down, it's over.' When you hit the tombstone, it's over. That's all we talked about."
Ultimately, Heidenreich came away from his time in WWE, which ended in 2006, with a very high opinion of The Undertaker.
"I still can't believe that I worked The Undertaker," Heidenreich said. "I think the world of him, and to be doing it as long as he has, I don't see how he still walks. I was in it 12 years, 13, 14 years. I have serious health issues, man. Some days I can't walk without a cane. Any athlete that goes professional in any sport, you're sacrificing."
The Undertaker had what was, by all accounts, the final match of his career at WrestleMania 33.