The Undertaker Reveals a Warning He Gave to John Cena Early in His WWE Career

Throughout the mid-to-late 2000s John Cena and The Undertaker were two of the biggest names in the pro wrestling industry. And while they didn't square off against each other very often, "The Deadman" still has nothing but praise for the 16-time former world champion. During a recent interview with After The Bell, Undertaker (real name Mark Calaway) talked about how he wasn't sure Cena would last as a top star at first, and even warned him about not getting burnt out on the wrestling business too quickly.

"I tell you what, when John came out early on, I would have never thought in a million years that he would get over to the extent that he did, and pleasantly, I mean just pleasantly surprised how over John got," Calaway said (h/t Wrestling Inc. for transcript). "I mean he was over over. There's that next level over that he was whether good or bad. That was another thing. I've never seen anyone last as long as he did with such a polarizing opposite love-hate.

"He put his time in. Man, I know John is a super overachiever, just his work ethic, second to none. I mean his media schedule on top of his work schedule. It was outrageous," he added. "I've actually told him that one time. I said, 'John, for your sake, you need to learn the word no because.' I mean he did everything. He never did. He proved me wrong, but I just said, 'man, you're going to burn yourself out.' He's amazing."

During a recent interview with ComicBook.com, Calaway explained why he and Cena didn't cross paths more often when both were full-time wrestlers. The pair only had two singles matches on pay-per-view, the latter of which was the three-minute squash match at WrestleMania 34.

"No, honestly it was just the booking," he explained. "And that time period when I was still working all the time, we were both babyfaces so you didn't really want to do that. And then it just came down to at the end there, John wanted to work with me and do something and it was fresh. And like I said, at this point in the career, it's hard to come up with fresh matches. It's just difficult because you have to cycle through everybody so many different times, but no there was never really reason why, it was just the way things were booked."