Thursday at 10:30 p.m., in the Madison Square Garden prize ring, the referee will tell both fighters to revisit their corners, wait for the opening bell, and come out fighting.

Only one of them will.

The reluctant warrior at that point will be, predictably, Muhammad Ali. He will circle and retreat, showing no enthusiasm for an early mix with challenger Earnie SHavers.

Any other tactic could be indiscreet and hazardous to Ali's health. Shavers, with those thunderbolts stockpiled in his Kong-sized, 18-inch biceps, has flattened 19 guys in the first round. He nailed 13 other venturesome opponents in round two.

Shavers does not like to risk late-round fatigue, which in his case always has been a problem.Thus his passion for short fights. As a knocker-out, his credentials are supergrade. Example: He has stopped 52 of the 60 men he has fought, mainly with his right uppercut, which some say is not only his best but his only skilled punch.

It is precisely because Shavers has this violent habit of knocking out almost everybody that makes his stalking of Ali's title fascinating. It is heating up as a prime-time fistic event.

The pragmatic betting is that Ali will finesse Shaver, take charge at will in the later rounds, and keep the title by which he lays such great store for its prestige and earning power. It may or may not be significant that while Ali is getting a $3 million guarantee for the 15 rounds or less, Shavers is coming in for a mere $300,000, only a bit more than the going price for Ali's most obscure opponents, like Jean-Pierre Coopman and Richard Dunn.

But there are factors that make it a high curiosity fight and that say that Ali could be in trouble in this one. He has never faced a puncher of Shavers' raw swatpower, including Joe Frazier and George Foreman, and at 35 he has other factors to cope with besides Shavers.

Only 10 days before the fight, he was looking puffy, and those bulges were not baby fat. The younger Ali was faster and could laugh at clumsy opposition, but he has lost much of his retreat speed and hand speed, and the fact is that in his most recent fights he has been huffing and puffing and unimpressive.

Ali is also indicating he is weary of the whole fight business, after 17 years in it as a pro. He has reneged on each retirement declaration because he still needs the big pay-days. His divorce was a financial bath. He took a huge loss in a Virginia real estate venture. His spending habits are so unabated that after his 1976 fight with Ken Norton. Ali's manager, Herbert Muhammad, was quoted: "I understand Ali used up his purse from the fight before he got it." To his dismay, his autobiographical movie, "The Greatest," is staggering at the box office.

Ali's training at his Deer Lake, Pa., camp has been a farce. He has become a regular on the black-tie circuit and is seemingly smitten with the importance of being White House dinner guest. He made it to Washington for the Panama Canal treaty-signing festivities and the next day was a breakfast guest at 1600 Pennsylvania.

Before and after, he was in New York making speeches for mayoralty candidate Percy Sutton and later switched to a survivor, Mario Cuomo, another loser.

Meantime, Earnie Shavers was in Ohio knocking over trees.

In the past, heavy punchers have not bothered Ali, Joe Frazier being the exception. Ali took away Sonny Liston's punching power by dancing and Jabbing until Liston opted out. He reduced Cleveland Williams and George Forman to nothings by outspeeding Williams before knocking him out, and trapping Foreman with his rope-a-dope before unloading the last punch of the fight.

But for Ali's admirers, the alarming truth is that he has been slow and clumsy in all but one of his last nine fights, the exception being his knockout of Frazier in those 14 grim rounds in Manila.

Otherwise, Ali was carried 15 hard rounds by Chuk (Who?) Wepner, looked slow and weary in beating Ron Lyle and Joe Bugner, knocked out such nobodies as Coopman and Dunn, lucked out in getting close decsions over Jimmy Young and Ken Norton, both of whom licked him but lost to the loaded peneils of the judges.

Maybe the one that could be pointed at as the absolute telltale fight was Ali's lost one, in May, at Capital Centre, Alfredo Evangelista not only went 15 rounds with Ali but thrwarted his punching power and footwork and dared him to come in an fight. In some minds, he deserved the decision over and out-of-condition Ali.

But it has been Ali's habit to win the big ones. Liston he knocked out, twice. Frazier he licked the second and third times around. Foreman he suckered and stopped to get his title back. And he also has the exclusive faculty of charming the New York fight judges, if necessary.

And counterbalancing what may seem to be the ripeness for an upset by Ali by Shavers is one significant historical fact: whereas Shavers lives by the knockout alone, Ali has never been knocked out in his life (54 fights).

At 33, Shavers is two years younger than Ali, 10 pounds lighter and three inches shorter. But curiously, he has an 80-inch reach, which is equal to Ali's and raw strength surpassing that of the champion. Shavers didn't get that way by eating spinach. He wrestled cars on a General Motors assembly line, worked the steel mills in Youngstown and stacked truck tires in Akron.

His huge, sloping shoulders and thick neck topped by a shiny bald head bespeak the power in those fists. Jimmy Young felt it one night. Shavers topped him in three. He has had knockout streaks of nine and 23, interrupted by his own knockouts by Ron Stander and Jerry Quarry.

It was in the Garden in 1973 that Shavers accomplished his most spectacular knockout. He was being punched around the set up by high-rated Jimmy Ellis in the first round when it happened, suddenlyu as described by this author: "Earnie Shavers brought a right upper cut up from the floor and Jimmy Ellis changed places with it." End of fight.

Shavers has what the boxing trade calls - and respects - "the puncher's chance." Not much of a jab but he can hit home runs. Dempsey, Marciano and Frazier were not stylists, either. But they had something nice going for them.