Watching a Wladimir Klitschko fight is like watching the Indy 500. You keep expecting a big crash.

That’s because Klitschko has become boxing’s latest sideshow, a large, polite, hard-punching fighter who can’t figure out why he keeps getting tagged by guys who aren’t suppose to be able to fight their way out of a knitting class.

Corrie Sanders did it first, tore off the cover and discovered Klitschko’s chin was made of mom’s fine china. He dropped Klitschko four times in two rounds in March, 2003. It was like watching a guy chop down a tree. Sanders won in two.

Then there was the Great Lamon Brewster Caper in April of last year.

Klitschko was humming along just fine in the early rounds when he suddenly lost all sense of balance and clear thought. That fight was over in five, with Klitschko looking like a drunk reaching out to steady himself before he does a header onto the barroom floor.

Since that loss, Klitschko stopped DaVarryl Williamson this past October, but a win over DaVarryl Williamson doesn’t bring you to the brink of the heavyweight championship.

That’s why Klitschko’s fight with Cuba’s Eliseo Castillo (18-0-1, 14 KOs) on Saturday (HBO) from Dortmund, Germany is so important. He wins that fight, he’s still a contender. If he loses that fight, he should seriously consider a career in medicine.

“I know a lot of people have a lot of doubts about me,” Klitschko said recently on a conference call from Germany. “But I don’t talk about that.”

Who can blame him?

Klitschko’s trainer, Emanuel Steward, knows what’s on the line in this fight.

He said Wladimir realizes this could be one of the most crucial matches of his career.

“He’s been training extremely hard for this fight and he’s very well prepared both mentally and physically,” Steward said.

One thing Klitschko (43-3, 39 KOs) won’t have working for him in this fight is his brother Vitali, the WBC heavyweight champ. Vitali was scheduled for back surgery earlier this week for a bone spur. And for the first time in Wladimir’s professional career, Vitali won’t be helping in his corner.

“It’s something unusual that Vitali won’t be in my corner for the first time in my 47 (pro) fights,” Klitschko said. “Physically he will not be with me, but mentally, he‘ll be there all my life.”

It’s hard to say if that will comfort him if he gets tagged in the fifth.

Steward said Vitali’s absence is really no big deal as far as his fight plan goes.

“When Vitali works in the corner, he’s a supportive brother but he doesn’t do a lot of talking,” Steward said. “I don’t think I’ve ever heard him say anything. He just supports his brother and sits down.”

As for Castillo, Steward sees it as a tougher fight than, say, a rematch with IBF champion Chris Byrd, who Wladimir beat in October 2000.

“I think Castillo is a much more difficult fight than Byrd or (WBA champion John) Ruiz,” Steward said. “Castillo is a much better fighter than Byrd. Chris gets hit a lot more.  Castillo uses his footwork. He moves around the ring very well and any mistake (Klitschko) makes, Castillo will take advantage of it immediately.”

Gentleman, start your engines.