ATLANTIC CITY, JAN. 20 -- "Perhaps in his mind he's very serious," said Mike Tyson, measuring his latest opponent, "but everyone knows that Larry Holmes sometimes speaks before his brain is in motion.

"I'm definitely confident of myself. I believe I'm a good fighter, and there's no one in the world who can beat me. I deal in facts, and my facts are beating everyone in the top 10. I can assure you of victory."

Coming back after a 21-month layoff, Holmes, 38, and for 7 1/2 years the heavyweight champion, has criticized Tyson for his manners -- Tyson refused to shake hands at a news conference announcing the fight -- and boldly predicted he can beat the unbeaten 21-year-old Friday night. To which Tyson, a heavyweight champion already being compared with the greatest in history, says, "I'm confident of my ability," and with that shrugs his massive shoulders and turns his 20-inch neck as if looking for some real threat to his title.

You couldn't see the ocean from the boardwalk today as fog covered the city. But for one more meeting with the media, which Holmes ducked, Tyson looked resplendent in his white silk shirt, blue suspenders with white polka dots and gray silk tie. He weighed 218 pounds -- "I'm in perhaps the greatest shape of my life."

A 7-1 underdog for the scheduled 12-rounder, Holmes was as invisible as the ocean, an empty chair marking his place. But like the waves, he could be heard -- so insisted Don King, a promoter with the chutzpah to size up the chair and interview the missing man, supplying both ends of the conversation:

King: "Larry, what kind of condition are you in?"

King: "He says he's in great condition. He says he's coming to fight."

King: "Larry, do you think you have any chance at all?"

King: "He says I'm going to shock the world. I'm going to show you, live and in living color, that I will be there. I'm not coming to lie down."

King: "Well, Larry, thank you so much for being here with us . . . "

Tyson, not given to laughter, had to laugh.Only in America

Mike Tyson is growing up. He jammed 28 fights into 1985 and 1986, once fighting three times in a month. Last year he fought four times -- making them count for millions. As King says, dusting off a phrase, "Only in America."

Tyson came out of Brooklyn, not the toughest guy in his neighborhood, but tough enough by the standards of the late boxing master, Cus D'Amato. D'Amato bailed Tyson out of reform school, adopted him as his son and turned him over to trainer Kevin Rooney, a former D'Amato fighter, and for managers, little Jim Jacobs, once the greatest handball player in the world, and dapper, 70ish Bill Cayton, he of thin cigars and French-cuffed white shirts.

On a dais today, Tyson and Cayton were sitting pretty and seemed to know it. If Michael Spinks -- himself undefeated -- wants to fight Tyson this year, Cayton said, he'll have to come to Tyson. "Without Michael Spinks, Mike Tyson will make over $50 million in the next year. Michael Spinks without Mike Tyson will make very little, if anything."

If Spinks wants to call himself "the people's champion," that's all right with Tyson. "I'm satisfied -- he can be the people's champion."

Only a monumental upset this week could unseat the one and only champion. Or a fluke -- Tyson admitted today he has been suffering bloody noses in workouts, but downplayed the matter as part of the business. "I've read some articles, and people have told me over the phone that reporters have said I was gaining weight and that I was fat. At 218 pounds, I'm solid as a rock. I can't take any more weight off."

Tyson has come down from seclusion of his own in Catskill, N.Y., unwilling to predict an early knockout of Holmes but sounding as if he would be happy to return to the quick-knockout format he used in '85 and '86. Last year, two of his fights went the distance (although Bonecrusher Smith backpedaled so fast he didn't stop to win a round); it took Tyson six rounds to dispose of Pinklon Thomas and seven to finish off Tyrell Biggs (but Tyson said he took his time because he wanted to make Biggs "pay" for prefight remarks).

In none of those four fights, however, did Tyson look less than unbeatable, boring in relentlessly, shaking off blows as if they were mere love pats, head dipped, shoulders squared, both fists working.

Every time he fights he takes it "very seriously" because "you never know what's going to happen. When you go see a comedy act, you know you're going to laugh. When you go see a horror movie, you go in there to be scared. When you go to a fight and you're rooting for somebody and it doesn't go your way, he gets knocked out or he loses, you leave crying, you don't want to be bothered anymore. Like I said, you don't know what's going to happen."

On Oct. 2, 1980, Tyson, who was 14 then, cried -- he cried because Holmes beat Muhammad Ali.

"I thought Ali was going to win."

As Tyson said, you never know about a fight.

Tyson can afford a lighthearted moment when he says of Spinks, "If he really wants to fight that bad, deep down in his heart, we'll fight for nothing, in a cellar."

But he's serious -- wearing the frown he does in the ring -- when he promises not to care if anyone says, in the wake of Friday night's fight, that he beat a "washed-up" champion in Holmes.

"Imagine if anything would occur, God forbid, and I lose to Holmes. I'd rather take the criticism for beating him than the criticism for losing to him."