By tradition, every U.S. Open must have a mystery man. Few Opens, however, can boast a third-round coleader as fascinating, yet only partially scrutable, as Japan's Isao Aoki.

In the past two seasons, Aoki has won 108,534,211 yen (217 per dollar at latest exchange) playing the Japanese golf circuit, while more than doubling that income with advertisements in a land that loves golf almost as much as it does baseball.

Nevertheless, outside Japan where he has won 32 tournaments, Aoki has left little mark on the game. In six Masters and three Opens he has played respectably, but never been a contender. Only in one World Series of Golf and one British Open has his game been among the leaders beginning the final day.

For three days in this 80th Open, Aoki has played head to head with Jack Nicklaus, shooting three 68s to share with Nicklaus the distinction of posting the lowest three-round total in Open history. He has done it with accurate, controlled driving and iron play, plus perhaps the most deadly putter in any Open going back to 1895.

"The putter is my trademark," said Aoki today, through a translator. "I have putted this way for 10 years," he added, demonstrating his unconventional style of putting with the blade at a 30-degree angle with the toe in the air.

Aoki's real trademark, however, is his temperament. Though he occasionally slaps the head of a club in the ground or grimaces while an errant shot is in the air, around the greens, he seems in a meditative trance of self-confidence.

"This is the most difficult golf course I have ever played," said Aoki. "It is very, very long. Much longer than any course in Japan. I must simply play along and be patient."

Does Aoki expect to rest easily in the Open lead?

"My hotel is very good," he said wryly. "I sleep very well every night and I expect to sleep well again."

Are his countrymen excited that he may win golf's most prestigious tournament? Will he be as famous as Japan's home run hitter Sadaharu Oh?

After hearing the translation into Japanese, Aoki smiled, then said in English, "Oh, no."

Is he aware of his pun?

"With Isao, you never know," said Lon Hinkle. "You think he speaks very little English, then he zings you."

Aoki, 37, began his interest in golf as a caddy, then turned pro in 1962. Despite standing six feet (an inch taller than Nicklaus), and weighing 170 pounds, Aoki is only a modest striker of the ball. His swing looks like a lunging, improvised version of Hubert Green's unorthodox slash, but Aoki's drives seldom have left the fairways here.

The uncanny gift that separates Aoki is his putting. At the 18th tee on Friday, Nicklaus and Littler added up Aoki's putts for the round. "We couldn't believe it," said Nicklaus. "He one-putted every green on the back nine and had 13 one-putts for the round. At every hole, Gene and I would say to each other, 'Well, he won't be able to get up and down from here.' And then he would."

Aoki may have the most unusual of all prescriptions for steady golf nerves. In Japan's candid press guide on its tour, Aoki's wife Hiroko (they have a daughter Joanne, 12) says she would only change one thing about her husband. "I wish he wouldn't drink so much," she said.

Aoki is legendary for consuming beer and a friend of his from Japan said "I have seen him drink a bowl of whiskey in two hours."

"How big a bowl?" he was asked.

"A Jack Daniels bowl," said Aoki's friend, pointing at a full fifth.

For Sunday's final Open round, playing for the fourth straight day with Nicklaus, there might not be any better prescription.