The first time John Tenta watched a Japanese sumo match five years ago, his observations and impressions were similar to those of others unfamiliar with Japan`s ancient national sport:

Two almost naked, barefoot and blubbery men wearing 33-foot-long Oriental G-strings called mawashi coiled around sagging bellies climbed into a circle of white, hard-packed clay 15 feet in diameter and for almost four minutes squatted, stamped their feet, glowered and tossed handfuls of salt into the air before finally charging at one another in a flurry of hand slapping, tripping and shoving until one was flung head over heels out of the ring.

The elapsed time for all this was 4 minutes 22 seconds--4 minutes of ceremony and 22 seconds of wrestling. And during the whole thing, a referee dressed in a 14th Century nobleman`s costume waved a fan and shouted incomprehensible descriptions of what was going on.

''I wasn`t really impressed,'' said Tenta, reclining in a dark blue kimono on a couch in the small office of Tokyo`s 200-year-old Sadogatake sumo stable, one of 30 currently training Japan`s approximately 800 professional sumo wrestlers. ''Obviously, my views on sumo have changed drastically since then.''

Indeed. Tenta, a 22-year-old, 6-foot-6-inch, 387-pound Canadian from the small lumbering town of Surrey in British Columbia, is on the verge of making sumo history in Japan. He has won all seven of his sumo bouts with ease and has one lower division championship to his credit--all in less than four months after arriving in Japan from Canada, where he was recruited by the Sadogatake stable.

As a result, many Japanese sumo experts believe Tenta, who goes by the sumo name of Kototenta, will become the first Caucasian ever to be admitted into the higher echelons of Japan`s highly stratified and tightly controlled sumo hierarchy.

That means Tenta, a former wrestler and football player at Louisiana State University and onetime junior superheavyweight amateur wrestling champion of the world as a member of the Canadian national team, would join the likes of the legendary Hawaiian sumo wrestler Takamiyama (born Jesse James Kuhaulua) and another Hawaiian behemoth named Konishki (born Salevaa Atisanoe) as the most successful foreigners to participate in Japan`s 2,000-year-old sport. Takamiyama retired in 1984. Konishki, who tips the scales at 505 pounds, is still active in sumo wrestling.

But before that happens, Tenta, who is still on the lower rungs of sumo-dom, must continue to prove himself worthy. And within the harsh, spartan world of Japan`s sumo stables, that is not going to be easy.

''There is a lot of pressure on me,'' said Tenta. ''The whole world is watching to see if a Caucasian guy can make it in this sport. I still have second thoughts about all this sometimes. But then I think, What was the alternative? I was going to become a professional wrestler in America. At least with sumo you are a highly respected athlete.''

But beyond the emotional demands, which call for total subordination to the oyakatta, or stablemaster, and a rigid pecking order which stipulates that newcomers like Tenta must wait hand and foot on the senior sumo wrestlers, is the brutal physical training that sumo wrestlers must endure.

''Nothing I have ever done--not football, not college wrestling--compares with the kind of physical abuse you inflict on your body in sumo,'' said Tenta. ''There`s no let-up. Not even when you`re injured or sick. That was really hard for me to accept. But I can already see that I am much stronger because of it.''

For the uninitiated, sumo training resembles what must have gone on in medieval European dungeons. Grown men shriek and weep in pain while the stablemaster whacks them on the back and legs with a bamboo rod if they don`t follow instructions.

''Are those tears?'' demands the sumo master.

''No, master, it`s just sweat,'' the sumo-tori lies, his face contorted in pain.

This, after hours of such exercises as teppo (slapping a round pillar of wood sunk in the ground with the open hand until the hand is swollen twice its normal size); shiko (lifting first one leg and then the other above the shoulder while in a squatting position and slamming the feet into the clay, often 500 times a day); and matawari (sitting on the ground with the legs spread at 180-degree angles and then trying to touch one`s chest to the earth. If a wrestler can`t do this, another wrestler will ''help'' him by climbing on his back until he does).

The day`s physical activity, which begins at 5:30 a.m. and doesn`t end until about 11 a.m., finishes with a flourish--an exercise called butsukari-geiko, or collision training, in which sumo wrestlers charge one another like enraged bulls, butting heads and bodies until flesh and bone are clapping in the stable like dry thunder.

After training comes the first meal of the day, a huge bowl of something called chankonabe--a high-calorie stew of fish, chicken, pork, seaweed, bean sprouts, carrots, tofu, cabbage and onions accompanied by equally huge portions of steamed rice, all of which is washed down with quarts of beer.

When they leave the stable, sumo wrestlers are expected to wear the classical male kimono and their oiled hair must be in the 18th Century chommage, or top knot, a cause of concern for Tenta because of his thinning, flaxen hair. Curfew is 10:30 p.m., and just about every activity inside and outside the stable is regulated by the powerful Japan Sumo Association, which promotes and demotes wrestlers and rules over the stables.

''To a lot of people back home, I guess sumo looks like a couple of fat guys with funny hairdos pushing each other around a ring,'' said Tenta, whose own hair, the Japanese press recently proclaimed, will more than likely be enough to create the traditional and mandatory sumo top knot. ''What they don`t know is that those fat guys are incredibly strong and they have amazing endurance, more than any football player or college wrestler. Nothing is as easy or as simple as it looks to the observer. There`s a lot going on in sumo, a lot of technique that the inexperienced observer misses.''

Actually, sumo aficionados point out that there are 70 officially recognized techniques that can be employed to defeat your opponent, which happens when you force him from the ring or cause any part of his body other than his feet to touch the ground.

''I`m still learning technique,'' said Tenta. ''It`s not enough to be strong. You have to know how to balance and how to channel your strength.''

If his seven victories, which Tenta dismisses as ''wins against kids,''

are any indication of his strength, then Tenta may be one of the strongest men ever to trod the hard clay of a dohyo, or ring. He dispatched his seven opponents in the grand total of 15 seconds, flinging them out of the ring the way William ''the Refrigerator'' Perry might deal with an opposing offensive line of Munchkins.