Unless old age arrives abruptly, Muhammed Ali will defeat Leon Sprinks in 15 rounds tomorrow night. It will be a workman like job, marked by none of the soaring art that make Ali unforgettable. Time has robbed him of his genius and left him a laborer; strong and brave, yet a laborer. Of Ali the youth, only the mouth remains. Listen. . .

"Leon Spinks is the Vampire," Ali said to a thousand people who paid $3 each to watch his work out the other day. "He is the Vampire because he doesn't have his front teeth and you see his fangs. I have told the world I will beat this Vampire if I stay out of clinches and don't let him bite me on the neck."

Ali's eyes widened at the dread thought.

"I might take a cross into the ring with me to scare off the Vampire," he said.

Better he takes a strong jab - and Ali knows it. He loves to perform for his worshipers, so he conjures up a Vampire. But this heavyweight championship fight is not act. Whatever Ali is - con man or evangelist, hypocrite or philanthropist - he is above all, a proud athlete. The nightmare of Ali's life is that he will be beaten badly in the ring, sent reeling from pillar to post.

That is why he will win the championship a third time.

He is too proud not to. Seven months ago, in against a kid who had only seven pro fights, Ali betrayed his gifts. By simply arriving in the ring, he believed victory was his. He trained a month, maybe two. Early in the fight, his strategy was plain.

As he had done so often in the previous two years, Ali would let the kid punch himself out. Let Leon Spinks get tired. The mental fatigue that attacks Ali opponents would get to Spinks. The physical fatigue would leave Spinks helpless for Ali in the late rounds.

That's the way Ali had been fighting everyone since George Foreman fell asleep in the eighth round in Zaire. If Ali were growing old, he yet retained astonishing stamina. Gone was the flash; in its place, Ali waged a war of attrition.

But Leon Spinks didn't get tired. And in the end, when Spinks had won a split decision, it seemed that Ali had given away the title by leafing through the early rounds. While that was true in part, it, too, was true that in the 14th and 15th rounds, with the decision still in doubt, Spinks was the dominant fighter.

That won't happen this time.

This time, Ali will fight well early, when he is at his quickest and strongest. He will rest an occasional round from the sixth through the 11th, as he must if he is to be ready for the end. No war of attrition this time; this time, if not Ali the artist, we will see an Ali of resourcefulness whose residual skills are good enough to handle Spinks.

"It's a mortal cinch Muhammad won't give away any rounds in this fight," said his trainer, Angelo Dundee. "That's just not going to work against this guy, because Spinks is a young lion."

Spinks is not a great fighter, but his is 25 years old with a body of rock and an animal eagerness for combat. Besides that, he is immune to Ali's psychological games (for the first time in memory, Ali is speaking kindly of an opponent, praising Spinks for giving him a rematch).

Before Spinks took Ali's championship, his courage in the ring was questioned. He flinched under attack. In with a mediocrity named Scott LeDoux, Spinks managed only a draw. He seemed a semi-pro stumblebum against an Italian named Alfio Righetti.

"I took him cheap because of that," Ali admitted. Well he might, and a question to be answered here is whether or not Spinks will revert to the LeDoux-Righetti form. If he does, Ali will jab him blind.

"Ali has to help himself this time," said Dundee, whose advice from the corner is sometimes heeded, sometimes not. "Remember, the last time Spinks just had a guy laying on the ropes, like a punching bag. Ali has to give him a little movement, make him chase him a little. That's all. LeDoux outstrengthed Spinks and Righetti outboxed him. And Ali is stronger than LeDoux and a better boxer than Righetti."

Prospectors who sift through an Ali training camp for nuggets of useful information nearly always come up with lead. For 18 years, he has been dreadful in training camp. Some witnesses this time say Ali has worked too hard, that he peaked three weeks ago and now is empty. They point to a miserable sparring session early this week as proof positive.

Dundee agrees that Ali was sorry that day. "He looked brutal," the trainer said. "And I told him so."

So the next day, Ali sparred without the protective headgear, a stratagem designed, he said, to make him less lazy in avoiding punches, thereby keeping him mind more on his work. His performance improved some, but hardly the "100 percent difference" Ali boasted of.

Not that it matters. Spinks, a little man next to Ali (195 to 220), is not a powerful puncher, and to win must do it over the 15-round route. Against an Ali ready to use his two decades of boxing wiles, the kid in his eighth pro fight is not a good enough boxer to win a decision.

But . . .

Always the: "but" of old age.

"Ali is in shape," said George Benton, one of Spinks' trainers. "But he is still 36 years old. Leon is in shape, too, and he is 25. You've got a 36-year-old fighter in condition and you've got a 25-year-old fighter in condition. Who would you take?"

The answer will come in, oh, the 14th round tomorrow night. More than anything else, Ali is motivated by self-made drama.It is important to him to be the first man ever to win the championship three times. What that superficial goal is added to the immense courage he has shown time and again, it will be enough to get him through a difficult 14th round and into the 15th, where he will win it strongly.