In a sense, the U.S. Open is the only golf tournament.

All the others, even the majors, are on a different and perceptibly lower level. Only the Open has everything.

The Masters may have the most blissful venture in golf and the blessings of an April date in Georgia. Also, the Augusta National is the best "action" golf course in America with many holes that reward a gamble and penalize cautious play; it's the place for streaks and collapses, for marvelous dramatic fun.

Unfortunately, the Masters, thanks to its ingrained elitism, has a small and weak field, diluted by doddering former champions, token foreigners and star-struck amateurs. Of the 100 best golfers in the world, the Masters may have no more than half in its field.

Add to this the undisguised biases of the course itself--it favors long, right-to-left, high-ball hitters, and you have a track where the same dozen names gravitate to the top of the leader board for years, even decades, at a time. Some of the game's best players--like Lee Trevino and Calvin Peete--have no chance to win.

The British Open has tradition, plus the quaint appeal of windy, links-style golf, an anachronistic 19th-century game full of weird bounces and improvised bump-and-run shots. If you love the smell of the sea and the sight of oceans of grass, it's lovely in its bleak beauty. And that's about it.

The British Open is also raw and frequently uncomfortable. If you aren't in love with the links look, the royal courses can seem like hopelessly boring eyesores. If the Masters fields are weak, then the British Open's are sickly. Dozens of top American players, who still constitute the overwhelming majority of the world's best performers, don't even bother to make the expensive trip. The B.O. remains a major by the skin of its teeth.

The PGA also has tradition and the spice of a new venue each year. However, the PGA often seems to be just a watered down, second-hand replay of the Open. The same set of great courses usually host both events with the important distinction that the PGA doesn't have the same courage of cruel conviction that the USGA maintains.

When the PGA sets up a historic course, as it did with Oakmont in 1978, the result is a difficult, but not really terrifying layout. Players here, such as '78 PGA champ John Mahaffey, freely maintain that there is little resemblance between a fair PGA setup and the sort of USGA chamber of horrors that pros remember with a shiver to their dotage.

Affairs like the Tournament Players Championship and Jack Nicklaus' Memorial Tournament are still just young aspirants in the great tournament category. The Memorial is really Nicklaus' attempt to mimic Bobby Jones and create his own May Masters; the TPC is, by utter contrast, a sort of anti-Masters.

That leaves us, thank goodness, with the U.S. Open, the tournament that players love to hate and fans love to love.

By traveling among all of America's greatest courses, the Open offers a fair chance to every sort of outstanding player during the course of his career. The man whose game isn't suited to Merion or Oakmont only has to wait for Pebble Beach or Winged Foot or Baltusrol to roll around.

The Open has the wisdom to maintain two absolute golfing constants. The USGA makes sure that every Open is a paramount test of a player's ability to use the game's two most definitive clubs--the driver and the putter.

The longest, and therefore most difficult swing in golf is the stroke taken with the driver. To hit the ball long and straight is the best test of a player's full swing. By making its rough the most fearsome on earth, the Open introduces a rich dramatic tension on every hole. From the time you put the tee in the ground, you're just a tiny muscle twitch away from disaster.

By ensuring that its greens are the hardest and fastest of any tournament, the Open introduces two more valid, though harsh tests. Only the most crisply and properly struck iron shots will hold Open greens; thus, the slovenly ball strikers, who can prosper on the soft hold-anything greens of many PGA Tour sites, are exposed as imposters.

Second, the sport's best test of nerves is the ability to endure four days of putting on greens where the tiniest misjudgement or mis-hit can produce a spectacularly ugly and embarrassing result. Hit a bad lag putt at the Doral or Kemper Open, and your ball may stop six feet from the hole. Do the same on Sunday at Oakmont and your dimpled coconspirator could end up off the green.

Golf only becomes a compelling athletic event--as opposed to an artful game--when the screws of tension are at their tightest and the physical demands for creative shotmaking border on the impossible. Only at the Open are great golfers consistently asked to execute shots that lay at the extreme outer limits of their ability.

A few statistics illustrate the mind games that great players are put through here. Through 36 holes, the top 10 players in this Open were hitting an average of only nine fairways and 11 greens in regulation, while requiring 30 putts per round. By a fine coincidence, these figures--nine fairways, 11 greens and 30 putts--are exactly the averages of the PGA Tour as a whole for the season.

In other words, the hottest players at Oakmont are performing at the same level as a group of mediocre players would on an average tour course. The average tour score for '83 is 72.4, while the field scoring average at Oakmont, through 36 holes, was 76.8. For comparison, the worst stroke average of any player on tour this year is 75.6.

Of course, no player can cope with this or any Open course for four days. Sooner or later, and often with mounting frequency as Sunday arrives, the mistakes arrive, the disappointments and frustrations mount. Thus, the final round of the Open is always about emotional and psychological survival, about facing the next test bravely even though you have just botched the last job badly with your whole world watching.

On Sunday, every contending player will find himself standing on his head in a church pew bunker, or discover his ball has burrowed to the bottom of a shin-deep clump of vegetation, or realize that he faces an unstoppable 20-foot downhill putt which will either go in the hole or else trickle 10 feet past.

That's what makes the U.S. Open golf's best and, in a sense, only championship.