NEW YORK, AUG. 26 -- If the ever-present heat, noise and louts in the bleachers don't get them first, there will be plenty of upset-minded opponents for the favorites to worry about at the U.S. Open. With hectic Manhattan as its backdrop, the last Grand Slam event of the season may be more eventful than usual given such unpredictability in the men's and women's fields as Steffi Graf and Stefan Edberg hold tenuously to their No. 1 rankings.

No one has been able to dominate this season, not Graf, with only her Australian Open title compared with staggering upsets at the French Open and Wimbledon, and not Boris Becker, who has been shut out at the major championships. The West German defending titlists are in uncertain moods, and moreover faced by rampant insurgence as no player among men and women has been able to capture more than one leg of the Grand Slam. So the Open may be a tiebreaker of sorts, the winners likely to be regarded as players of the year.

Rarely have the top seeds been so closely grouped approaching the Open, which begins Monday with first-round play at the National Tennis Center in Flushing Meadow and promises, just perhaps, some hard answers to the confusing season. Or perhaps not. It could go much as the rest of the year has, with startling turns of events such as the French Open victories of Andres Gomez of Ecuador and Monica Seles of Yugoslavia, Zina Garrison's upset of Graf to reach the Wimbledon final, and John McEnroe's fade, unseeded for the first time in 12 years but still good for a tantrum or a shock.

"The race is great for tennis," fourth-ranked Garrison said. "But the 1-2-3 thing may not matter. There are 15 people who could win the Open."

Six players have claimed the three previous Grand Slam events, the most divided season since 1987, when Hana Mandlikova and Edberg won the Australian Open, Graf and Ivan Lendl won the French and Navratilova and Pat Cash won Wimbledon. Navratilova and Lendl emerged as Open champions to clarify that situation.

And it will be a quieter Open since air traffic controllers at LaGuardia Airport have promised to keep jets off Runway 13, preventing them from roaring directly over Louis Armstrong Stadium.

Sweden's Edberg, 24, seized the No. 1 ranking for the first time in his career on Aug. 13 by virtue of a winning streak that included his Wimbledon title, No. 2 Becker's inconsistent form and No. 3 Lendl's long absences from the tour.

"At the moment, I feel like I'm No. 1," he said as he stepped into the ranking. "It's very important to me. It will be interesting to see how long I can stay there."

The stoic, 30-year-old Lendl was a model of consistency at the top, 80 straight weeks as No. 1 before he was deposed. But he devoted most of the season to preparing for Wimbledon and has no mathematical chance of regaining the top spot even with an Open victory. Still, he has made eight consecutive Open finals to tie Bill Tilden's record, and may yet interfere with that rivalry of the future between Edberg and Becker, 23.

"Ivan, you know, he was eight times in a row in the finals," Becker said. "So I think he is the guy. Stefan was there twice in the semifinals. . . . He has a chance."

Of the three, Becker has by far the most difficult draw. His first-round opponent is Juan Aguilera, who beat him earlier this season. Things do not get easier: Potentially, he has a second-round match with Yannick Noah of France, against whom he is 2-2; a third-rounder against Paul Haarhuis, who upset John McEnroe last year; and a round of 16 encounter with Goran Ivanisevic, the sensational teenager who beat him in the first round of the French and nearly did so again in the Wimbledon semifinals.

Edberg also has a hand-wringer in his opening match, against Alexander Volkov of the Soviet Union. Lendl has by far the easiest draw, with a qualifier in the first round and no one else of note until the quarterfinals, when he could meet Thomas Muster.

Complicating the men's side are two declining former champions in McEnroe and Mats Wilander of Sweden. McEnroe, ranked No. 21, may have difficulty in his opening-round match with resourceful, young Javier Sanchez of Spain. Wilander rose to No. 1 with his 1988 Open victory but has been idle much of the season to plunge to No. 58, and must meet eighth-seeded Brad Gilbert.

The women will contest a tournament that is anybody's guess, each of the top three players given equal chances, and an interloper like Garrison also possible. Graf, 21, has been unassailable for three years, but now is faced with 33-year-old Navratilova's strong resurgence with her record ninth Wimbledon title, and a driven new challenger in two-fisted, 16-year-old Seles. Fourteen-year-old Jennifer Capriati, who gave Graf a three-set struggle before losing an exhibition today in Mahwah, N.J., is another pony-tailed player who could contend.

While Graf cannot mathematically lose the No. 1 ranking as a result of the Open outcome, the year-end No. 1 would be in jeopardy. Quite simply, Graf must win the Open to remain the best player in the eyes of the tennis world.

"Whoever wins the tournament between Martina, Steffi and myself will probably be regarded as the best player of the year," Seles said.

Seles has amassed a 43-3 record with seven titles in 11 tournaments. Her head-to-head mark against Graf for the season is 2-0, against Navratilova, 2-1. The case for Navratilova is as follows: a 45-4 record, six titles, and the lead in prize money. She has also won tournaments on four surfaces, an accomplishment Graf and Seles have not equaled.

As for Graf, it is a statement about her lengthy dominance that a year such as this one, with only one Grand Slam title, is viewed as such a downfall. The 1988 Grand Slam winner has been plagued by a sinus condition and tabloid allegations about her father's personal life. Weakened by her painful condition, she was upset by Seles in their Paris final, and, hounded by newspaper stories, was stunned by Garrison in the Wimbledon semifinals. "Every day there were things going on," she said. But the publicity furor has quieted, and the day after her Wimbledon loss she flew home to West Germany for a minor sinus operation. She arrives in New York in a better frame of mind.

"After all I've gone through this year, I'm fine," she said. "I think I should be eager for the Open. I've gotten it all behind me now."MONDAY'S FEATURED MATCHES Stadium Court

Elena Pampoulova, Bulgaria, vs. Monica Seles (3), Yugoslavia; Wally Masur, Australia, vs. Jim Courier (14), Dade City, Fla.; Steffi Graf (1), West Germany, vs. Maureen Drake, Canada; Juan Aguilera, Spain, vs. Boris Becker (2), West Germany; Brad Gilbert (8), Piedmont, Calif., vs. Mats Wilander, SwedenGrandstand Court

Thierry Champion, France, vs. Tim Mayotte, Bradenton, Fla.; Mary Joe Fernandez (8), Miami, vs. Ann Henricksson, Mill Valley, Calif.; Pete Sampras (12), Rancho Palos Verdes, Calif., vs. Dan Goldie, Redwood City, Calif.; Manuela Maleeva-Fragniere (9), Switzerland, vs. Amy Frazier, Rochester Hills, Mich.; Yannick Noah, France, vs. David Pate, Las VegasOther Seeded Players

Court 3: Aki Rahunen, Finland, vs. Thomas Muster (6), Austria

Court 6: Andrei Chesnokov (10), Soviet Union, vs. Olivier Delaitre, France

Court 12: Martin Blackman, New York, vs. Martin Jaite (16), Argentina

Court 16: Jo Durie, Britain, vs. Helena Sukova (11), Czechoslovakia

Court 18: Mary Lou Daniels, Chicago, vs. Judith Weisner (15), Austria