TOKYO — Playing the jester on the ropes, Muhammad Ali Saturday pounded pretender-contender Mac Foster into sightless, stumbling wreckage to win a unanimous 15-round decision and retain his right to challenge the only man who ever defeated him.

It was April Fool's Day and many Japanese and foreign fans in the Budokan felt they were watching a grisly joke. — a lopsided mismatch}between a boxer too good to be beaten and a fighter too tough to go down.

Ali lanced Foster's stolid features with popping left jabs, cruel right crosses and crushing hooks. He hit him with everything but his cornermen's bucket. But as the final bell tolled the fight into the record books, there was Foster — fighting in a vertical coma but still on his feet.

The decision was beyond doubt. Referee John Crowder, an Air Force technical sergeant assigned to Fuchu Air Station, called it for Ali 73-65, while Judge Hiroyuki Tezaki had it 7465 and Taekeo Ugo 75-67. UPI scored it 74-64 for the former heavyweight champion and AP 73-64.

The Pacific Stars and Stripes card, on which Foster did not win a round, had it 70-59.

Yet Ali's bombastic prefight prediction — "Round five 'cause he's talkin' jive!" — failed to materialize. Foster was never even on the floor. Unmarked but slouching in round-shouldered exhaustion, Ali rested for 15 minutes before he held court for newsmen, praising his defeated opponent and displaying none of his usual flamboyance.

Ali is still No. 1 contender for the heavyweight championship held by Joe Frazier, who beat him on March 8, 1971, for the only loss in Ali's 35-fight career. He now hopes for a September rematch that could bring both he and Frazier more than $6 million. He must first get past George Chuvalo and Jerry Quarry, both of whom he has defeated before.

The bout began like a fashion show. Foster, wearing a light blue robe, was in the ring first. Then a spotlight haloed Ali as he strode to the ring carrying a sign with a large number five. Ali wore a multi-colored robe sewn with dragons and other gaudy designs. Foster followed Ali as he circled the ring with the sign, acting as though he was going to give him a paternal pat on the head.

The pre-fight showmanship was at last put aside. The bell — and a rocky first round for Foster. He rushed Ali and missed a sweeping hook. Ali, forcing the fight, thrust in several punishing jabs and hooked the left, turning Foster's head and knocking a spray of sweat into the overhead ring lights. He moved in and out with blinding speed, slipping all but a couple of blows that grazed but did not stun, thumping home short punches to the heart and head.

In the second round, Foster landed a good right high on the head and blasted hard at Ali's body. Ali tottered gracefully, took it all and once defiantly dropped his hands. He effortlessly threw back damaging return fire.

The third brought little change in the pattern of the fight — except that Ali lolled briefly on the ropes and appeared to be deliberately soaking up body punches to show how powerless Foster was. The fourth saw Ali land a right that brought a remote, faraway-places look into Foster's eyes. But Ali himself seemed flushed and shaken a moment later as he paused on the ropes to clown — perhaps to stall.

Big Five — no Ali jive, at least none that brought a knockout and fulfilled a prediction. Ali lunged with a right that shook Foster and flung over combinations that landed with cracking thumps. Another right half turned Foster around and lifted one foot off the floor. Foster was groggy but not out. He stung Ali with a few of his own. The bell rang, and Ali grimaced and spread his gloves in a gesture of self-disgust.

In the sixth, Foster raised his guard, absorbing plenty of punishment but denying Ali a facesaving KO. He lunged at Ali's body the next round and Ali laid over both of his shoulders and took all he threw — an act repeated many times. He was pushed to the ropes and into corners — he didn't care and no punch even made him breathe hard. "Look," he seemed to be saying to the crowd, "look upon this youth, this upstart, this pretender who would move up from ninth in the ratings and sit upon my summit. I feel nothing. He hurts me not."

Ali never taunted him aloud — only with his fiendish skill. Again and again, he thumped Foster into near oblivion — again and again, Foster slipped away. And suddenly, Ali was jeered from ringside to gallery. Was he punishing instead of finishing? Was he carrying Foster — testing his durability at the expense of an opponent he obviously outclassed in every way?

Cheers began to sound for Foster — fans began hailing gallant effort instead of spectacular performance.

Like some terrible form of evolution, ledges broadened over Foster's eye and along his upper cheek. He wasn't cut but his nose bled a lot. As he desperately wheezed for breath, a crimson spray flew. Ali's white, red-striped trunks looked like a battle surgeon's smock.

The fight wore on in monotonous pattern. Ali's jab was a darting bludgeon. He landed this hook, that right cross a shattering combination. There was Foster, still maddeningly erect; there was Foster, his eyes like knife slits, making a turn-backthe-clock KO effort in the 14th. There was Foster, blindly lumbering after the fleet Ali and trying to stalk and corner him as the final bell rang.

Ali, Foster reported later, embraced him and said: "Goddam, you're a hell of a son of a bitch" — a tribute to Foster's valor and his own flustered attempts at a knockout.

In the dressing room press conference, with Foster sitting beside him, Ali said his vanquished opponent was a "great fighter" because he was good enough to stay in 15 rounds with "the greatest."

"I had a good combination but couldn't quite get it off because he had an extremely good jab," Foster said. "He is absolutely the best and I don't feel bad about losing to him."

Ali said he was foregoing his usual postfight victory party. "He took a lot out of me. He's a good fighter. I couldn't put him out in the fourth because his defense was too good and after I couldn't fulfill my fifth round prediction, he covered too well. I gave up trying."

Angelo Dundee, who trains Ali, threw a sympathetic arm around Ralph Gambina, who handles Foster. Foster himself had no alibis but his manager, George Stassi, said he had been troubled for 10 days by pains in his left elbow, injured before he was stopped by Jerry Quarry in 1970 — his only other loss in 32 bouts. In his own dressing room, Foster appeared to have trouble straightening the arm out.

"Where's that five?" Gambina asked repeatedly. "We went three times that five."

Ali also said he will be in Japan for a few more days to finish a film documentary. He confirmed he is not going to mainland China from here because, as a Black Muslim, he cannot travel without the permission of Elijah Muhammad and forgot to get it before he came. Robert Arum, his lawyer, gave upcoming fights and a possible Frazier rematch as the reason and said the visit had only been postponed — not cancelled.