Only the best and bravest will ever know the bizarre mixture of elation and frustration that Greg Norman felt this evening.

The Great White Shark stalked off Shinnecock Hills, jaw clenched in anger, after shooting a glorious 68 to take a three-shot lead over Lee Trevino and Denis Watson at the midpoint of this U.S. Open.

Almost any other golfer on land or sea would have been delighted at equaling the second-lowest score of a warm, windless, heavenly afternoon. But Norman knows that, after shooting 31 on the front nine, he had a five-shot lead and could have stuffed his championship in his maw and gobbled it.

"I'm happy, but also disappointed," said Norman, who is the No. 1 money winner on the PGA Tour after a fabulous streak of two wins and two runner-up finishes that began with his five-birdie binge on the final nine holes at the Masters and extended through his victory in the Kemper at Congressional. "I could have had a better cushion . . . 68 is the worst score I could have shot."

By staggering around the back nine in 37 blows for a two-day total of 1-under-par 139, Norman allowed an all-star cast of tough customers to join in stern pursuit. At 143: hungry sophomore sensation Bob Tway, a winner last week and the leader Thursday; ravenous and confident Tom Watson, who hasn't won for two years; and famished Ray Floyd (68), who's not only zero for 22 in U.S. Opens but has never even cracked the top five.

As if that weren't worry enough, Bernhard Langer, Lanny Wadkins and brother Bobby, and the strictly individual Mac O'Grady are part of the logjam at 144. Throw in Ben Crenshaw, Craig Stadler and Hal Sutton -- all major-title holders -- at 145 and you have one of the most glamorous Open leader boards ever. Shinnecock Hills has brought the cream to the top.

Norman knows he could have made this a very dull weekend.

As he came to the 10th tee, with flags limp and a cloudless sky, this old links was suddenly as defenseless as it had been fierce the previous day. Now was the moment for a decisive Shark Attack. As the sun sank, birdies started dropping all around Norman. Danny Edwards (then 17 over par) shot 30 on the back side (and missed the cut). Joey Sindelar (then 11 over) scorched the back for 31 and a course-record 66. Et cetera.

For Norman, 31 and fabulously gifted, but bereft of any major title, only gremlins lay in wait. A poor club selection at the 10th led to bogey. A moment's lost concentration on the green squandered a birdie at 11. A rude cameraman's click probably ruined a four-foot birdie putt at the 12th; certainly it infuriated Norman, who chewed the fellow out. At the 13th, rough and sand led to a bogey. The magic was lost.

While others were knocking down flags, Norman was two-putting his way to the house from 30- to 50-foot range over the final five holes. Was this the same daring man who needed just nine putts on the front nine? The chap who chipped in from the fringe to save par at the sixth and made birdies at the first, fourth, fifth and eighth holes?

Generous as Norman was at the close of the day's business, the consensus among contenders here is that this is the glamorous Australian's tournament to win or lose. He's at the point where his peers stand back and give him room.

"I'm going to be awfully surprised if anyone catches Greg. At this point in his career, he exemplifies the aura of being a true virtuoso," said O'Grady (69), the pro who only overclubs when he speaks. "Right now, he's playing another type of game . . . He has sinews of character deep within him."

Yeah, and he also hits the tee ball about 320 yards down the watering system.

Perhaps Floyd, 44, sees Norman most accurately because 20 years ago, he had almost identical gifts. "Norman thinks that nothing can go wrong," said Floyd, speaking of the confidence of youth. "He has all the tools to win the Open. Even more skills than necessary.

"He outdrives my best ball by 40 yards and, normally, it's more like 70 yards," said Floyd. "Twenty years ago, I was as long, or longer than Norman. But you can't go back."

To get a feeling for Norman's mystique at this moment, listen to Floyd talk greedily about what youth was that age has lost.

"As you get older there's this thing in your head that clicks and says, 'You shouldn't do that,' " said Floyd after both he and partner Trevino had shot 68. "When you're young, you don't have it. You go out there without doubt or fear. And that makes you longer and stronger. You can do awesome things. Go through trees, around corners, out of deep grass, hit it 330 yards on the fly.

"Now, I'd say those things couldn't be done. I can't even fathom it. But I know I did them. I remember."

With age, with failure, you learn all that can go wrong. As Floyd says, "Fear is a great intimidator."

For the moment, Norman still has no fear. At the 1984 U.S. Open, he made a 40-foot putt on the 72nd hole to force a playoff that he eventually lost to Fuzzy Zoeller. This year, he ran down Jack Nicklaus at the Masters, but missed a playoff when he bogeyed the final hole.

As yet, the near misses don't haunt him. Norman, fresh from his come-from-behind victory at the Kemper Open, only wants the next meal, the next great shot. Yes, the next conquest.

"It was just a joy to play out there," said Norman, who was plagued for a year by a mysterious strength-sapping lung infection that was diagnosed and apparently cured only in the last three months. "I think I'm a better player now than at Winged Foot in '84 . I'm a lot more of a streetwise golfer. By playing the American tour, I've learned a lot about my game and about myself."

While Norman would make a real shark look as if it lacked confidence, the second-place fellows in his wake are not exactly carrying their clubs like swagger sticks.

"I'm happy. It's the first time I've played any good all year," said Denis Watson, 30, a Zimbabwean. "Jack Nicklaus helped me last month at his Memorial Tournament. We played together the first day. I hit the ball, if anything, better than he did. Jack shot 66. I three-putted five times, four-putted once and shot 76.

"As we went down the last hole, he said, 'I have a spare putter. I really think you should hit some putts with it,' " said Watson of Nicklaus' now famous superlarge, war-club-sized putter. "Obviously, it's helped. He's always been one of the nicest, most generous people I've ever met."

Also, one of the more chapped after his 77-72 -- 149 work here made the cut with but a stroke to spare. "Not a great score, obviously," said Nicklaus, chewing up ice cubes as though to punish himself.

Those who missed the cut include Tournament Players Champion John Mahaffey (73 -- 152) and 1985 leading money-winner Curtis Strange (79 -- 155).

Seve Ballesteros stayed alive at 73 -- 148. British Open champion Sandy Lyle was 71 -- 149. PGA champion Hubert Green and defending U.S. Open champion Andy North were at 150.

NCAA champion Scott Verplank assured himself a paycheck in his first tournament as a pro with a 147 total. Past Open champions Larry Nelson and Johnny Miller joined Andy Bean and Ballesteros in the group at 148.

The day's happiest 46-year-old was not the Olden Bear, Nicklaus, but Trevino. "I can't wait for tomorrow," said Trevino. "I'm playin' great. Don't know how confident I am, but I'm hittin' it great for now.

"I absolutely fell in love with this course when I saw it on Tuesday. It doesn't favor any one type of player."

Translated from golfese, this means that Shinnecock Hills doesn't favor any type of player other than Trevino's type. This is a tight, midlength faders' course with high rough, links wind and less than lightning-fast greens, which means it is the definition of a Trevino course.

One unfortunate reporter asked Trevino about his chances in light of the fact that he hadn't won a major championship in so long.

"You musta been in prison in 1984," Trevino snapped. "I won one of these not too long ago the 1984 PGA . . . If I play the next two days like I played the last two days, I got a heck of a chance to win."

But not the chance that Norman has. "I'm not out here to be a gallant second place. I'm here trying to win," said Norman. "I'll keep being aggressive. Even a 10-shot lead's not comfortable. You always play like you're three shots behind."

Of course, it's everybody else who's three or more shots behind.

And lucky at that.