The 48-year-old Holyfield (43-10-2), who held the world heavyweight title four times, arrived in Denmark on Sunday for the bout at Copenhagen's concert hall and said he wouldn't be fighting if he wasn't ready.
Holyfield's latest bout in January against Sherman Williams was ruled a no-contest. Before that, he knocked out Francois Botha after losses to Nikolay Valuev and Sultan Ibragimov.
"If I win, I am moving to the next level," Holyfield said. "I am hoping to get a championship bout with other guys with other belts.
"Back then when I was young, I made a lot mistakes but I had the stamina and all that to overcome. Today, you have less stamina but more focus. Either way you go, if you play with what you have, you win."
Nielsen (64-2, 43 KOs) acknowledged Holyfield is the favorite but promised the fight won't be a walkover.
"I know that he will be really tough to beat, but he won't meet someone who will throw himself onto the floor before him," Nielsen said after a training session. "I will fight until I cannot stand up any longer.
"I have fought 200 fights and I have never been knocked out, so why should that happen now?"
Nielsen, who lost to Mike Tyson in 2001, last fought in 2002 and retired soon after because of a knee injury.
The Dane won the lightly regarded IBO title in 1996 when he stopped Tony La Rosa in the second round. He also ended 47-year-old former champion Larry Holmes' hopes of a winning another belt in a successful title defense a year later.
"It's two old boxers meeting. We are getting older, there's no doubt about that," Nielsen said. "It is an old boys fight, but I believe we can give more than some of the youngsters can."