Novelist J.P. Donleavy once wrote of the umpirees at Wimbledon that they "exude rectitude in all directions." One would expect nothing less of the men who wear the blue blazers of authority at the world's oldest and most revered tennis tournament, for the All England Lawn Tennis and Croquet Club is a fortress of British reserve, decorum and dignified understatement. From the dapper chap who oversees the manicuring of the lawns to the pin-striped committeemen with roses in their lapels, Wimbledon itself exudes rectitude n all directions.

Not so the U.S. Open.

For one thing, the sauna-like late summer heat and humidity of Flushing Meadow is not conducive to te wearing of blazers. The six-person officiating crews that have replaced the customary 13-member contingents at the National Tennis Center -- one umpire, two seated baselinesmen, two roving sidelinesmen and one service linesman who alternates sides are being used this year in an effort to upgrade officiating -- are decked out in pale blue knit jerseys, gray slacks or skirts and sneakers.

They have to be on their toes, moving, because the official behind the baseline must call the center service line at the start of a point, then scurry into position to judge sideline calls on both sides of the net. There is no time to stand around and exude rectitude in any direction.

Wimbledon and the U.S. Open are mevitably compared because they happen to be the two most important tennis championships in the world. But they are as different as strawberries and big apples.

Wimbledon is hydrangeas and coddled grass courts with a middle-class garden party tradition of strawberries and cream. "The Open," as America's premier tournament is commonly known, is plastic geraniums and skidmarked asphalt courts and cheesecake, providing distraction from the players slogging and sweating, grunting and cursing, amid the heat, the smog and the ceaseless noise.

Distractions are an integral element of the Open, part of its character and challenge. To win here, a player must be able to concentrate through chaos. He must accept a certain maddening level of commotion and insanity and rise above it, relying as much on will as skill.

The Open is played in a public, city-owned facility in Flushing Meadow park, site of the 1939 and 1964 World Fairs. Planes roar deafeningly overhead from nearby LaGuardia Airport. Ice cream trucks cruise the streets outside, ringing bells and playing kiddie jingles, not 30 feet from some of the outside courts. The aroma of hamburgers charcoal-broiling in concession tents and corporate entertainment marquees wafts across the courts.

Spectators scream and move around as they wish. If they feel like buying a hot dog and a beer, they get up and fetch them, whether a point is in progress or not. They could not care less about gentility or tennis manners, which, as far as New York is concerned, went out with white flannels.

The players complain about the heat, the pollution, the noise, the confusion and the courts that are hard on the feet, even when they are not sticky from spilled soft drinks that were never wiped up. But New Yorkers love this tournament. It is a spectators' event, and they have taken it to heart. They can identify with impolite patrons and high decibel levels. Like the Big Apple itself, the Open is a testament to the fact that great things can come out of claustrophobia and rudeness.

At Wimbledon, everything runs smoothly, like clockwork. At Flushing Meadow, even the clocks don't work. When Bjorn Borg -- the five-time Wimbledon champion who has never won the Open -- went onto the grandstand court today at 2 p.m. to wipe out a hapless Argentinian named Guillermo Aubne in his first round match, the courtside clock with a "timex" promotionhal message read 9:20. It rapidly advanced by nine hours, and never did settle on the correct time.

At the start of the match, neither of the electronic scoreboards were working. By the end, one was. The umpire's microphone never functioned. Even if it had, his voice probably would have been drowned out, because the takeoff pattern at LaGuardia was sending planes directly over Flushing Meadow every 90 seconds, seemingly close enough to reach up and touch.

Meanwhile, next door in the 19,000-seat Louis Armstrong Stadium, defending champion John McEnroe was beating up on Frenchman Christophe Roger-Vasselin. This was the same McEnroe who last month took Borg to 6-4 in the fifth set of a majestic Wimbledon final, one of the best and most thrilling tennis matches ever played, but today his hometown spectators were jeering him as his celebrated temper boiled over in the 90-degree heat.

"I feel I've gotton a better reaction from the public since Wimbledon, but you can't expect one match to turn it around," said McEnroe, who earnestly wants to bury his image as a hothead prima donna. "People were egging me on today, and when it's so hot, you have a tendeny to shoot back a lot faster than you would otherwise. That's a problem here. And New York crowds are really fickle -- they'll give you a big hand one point and they're booing you the next."

McEnroe seems able to channel hostility and use it to his advantage. He thrives in an atmosphere of villainy, even though he dosen't like it, much as Jimmy connors did in his heyday.

Perhaps that is why Connors won the Open three times -- 1974 on grass, 1976 on clay, and 1978 on the current hard courts -- while Borg, a better player, has won five French Opens and five Wimbledons but no U.S. Open to date. Connors, a street fighter like McEnroe, feeds on the free-form madness and untamed energy that is Flushing Meadow. Borg is strong, quiet, composed, and elegant in his machine-tooled superiority. Like Wimbledon.

Borg and McEnroe both proved in that marvelous, marathon Wimbledon final that they are grand champions who don't know how to quit. New York fans hope that despite Borg's tender knee and McEnroe's sprained ankle, both of which seemed insignificant today, they reach the singles final Sept. 7 and provide an encore.