The oppressive heat and humidity that had turned the first three days of the U.S. Open tennis championships into a communal steambath gave way today to mid-70-degree temperatures and a pleasant breeze. Of the favorites who played, only Martina Navratilova had to sweat. But Ilie Nastase got hot under the collar -- which could happen in the Arctic -- and was assessed $500 in fines during a pathetic loss to Harold Solomon.

Navratilova, the two-time Wimbledon champion who has never won the Open, hates almost everything about the tournament, and says that "if it wasn't so important, I'd avoid it at all costs," struggled to a 6-1, 4-6, 6-2 victory over New Yorker Leslie Allen in a comedy of unforced errors that was so bad it had to be seen to be disbelieved.

That was the day's first match in the 19,000-seat stadium of the National Tennis Center, which was sold out on a weekday for the first time. It may have been bad tennis, but what followed was bad burlesque.

Nastase, who had been playful in his first-round victory over Patrice Dominguez and still accumulated $750 in fines for misconduct, played dreadfully and behaved worse in losing to No. 7 seed Solomon, 6-2, 6-2, 6-2.

Nastase was fined by Grand Prix supervisor Kurt Nielsen for twice throwing balls at chair umpire Ken Slye of Panama City, Fla., with whom he argued frequently. Slye twice docked Nastase penalty points for stalling and using abusive language, once on a game point.

Defending champion John McEnroe and No. 3 seed Jimmy Connors, who ruled the Open in 1974 on grass, 1976 on clay, and 1978 on the current asphalt-based hard courts, won their second-round matches in straight sets. McEnroe beat Steve Krulevitz of Alexandria, Va., 7-6, 6-0, 6-2, and Connors routed Butch Walts, 6-3, 6-1, 6-3.

With last year's runner-up and No. 5 seed Vitas Gerulaitis out, beaten by Hank Pfister in a fifth-set tie breaker on Thursday night, Bjorn Borg and Guillermo Vilas look like good bets to reach the semifinals in the top half. There are more "spoilers" in the bottom half -- including Solomon, Brian Gottfried (a 6-3, 6-2, 7-6 victor over Tim Gullikson today) and Ivan Lendl (conqueror of Chris Mayotte, 6-4, 6-4, 6-4) -- but McEnroe and Connors seem in form to reach an anticipated showdown in the semis.

McEnroe took a set to adjust to what he called the "strange conditions" of the 7,000-seat grandstand court -- the movement of spectators entering and exiting the adjacent stadium, the slope and pace of the surface -- but he served and volleyed exceptionally well the last two sets.

Krulevitz, the affable 29-year-old Baltimore native who plays Davis Cup for Israel, also played above himself in that first set, getting in consistently deep first serves, volleying confidently and ripping some superb return winners off McEnroe serves.

There were no service breaks in the set, and Krulevitz looked inspired as he got to 5-5 in the best-of-12-point tie breaker with a fierce overhead smash. hBut then McEnroe hit a good forehand, down-the-line pass for 6-5, and took the set with a sliced serve deep to Krulevitz's backhand that opened up the court for a simple volley. That point set the pattern for much of the next two sets.

Krulevitz played just about as well as he can in that first set, and still lost it. That had to be discouraging. Thereafter, he missed his first serve with increasing frequency, and McEnroe began feasting on the second -- chipping or bashing it and darting to the net for killing volleys, or forcing Krulevitz to rush and miss his passing shot attempts.

Then again, Krulevitz -- No. 104 in the latest computer world rankings -- didn't expect to beat the reigning champion. If he had, he wouldn't have scheduled his wedding to Ann Gorneva of Alexandria for next Saturday: men's semifinal day at the Open.

"He started to play better in the second set and put more pressure on me. I think he got a little confidence in the tie breaker," said Krulevitz. "He wasn't hitting the ball that well in the first set, but his serve is still tough. Very tough to read. And he hit some good, deep volleys at crucial times."

McEnroe continues to play with both ankles tightly strapped to avoid a recurrence of recent strains, but he is playing the best he has since his five-set loss to Borg in a marvelous Wimbledon final. Still, he is wary about taking a rematch with Borg, Sept. 7, for granted, as so many people seem to be doing.

"I feel if I play well, I have as good a shot as anybody," said the 21-year-old lefty from Douglaston, N.Y., even though he thinks the relatively slow, high-bouncing courts at the resurfaced National Tennis Center favor Borg over the native sons.

"People say it'll be a Borg-McEnroe final, but there are a lot of players. I'd be happy just to go out and play him in the final, but you have to go through the whole draw, and the pressure builds as you do."

With all the attention being focused on Borg and McEnroe -- understandably, considering the masterpiece they produced at Wimbledon -- Connors is rather the forgotten man of this Open. But he has always played well in New York, as if energized by its frenetic atmosphere, and today he was hitting the ball sweetly against Walts, the strapping, 6-foot-4 Warren Beatty lookalike who dethroned Vilas as Open champion in a memorable five-set match here in 1978.

"I just go out and play tennis. I don't care if they write about Borg and McEnroe and don't write about me. It doesn't bother me," said Connors, who at 28 has mellowed off the court but remains an uncompromising competitor, powered by a furious internal combustion, on it.

"If I go out and play the kind of tennis I'm capable of playing, they'll have to write about me," he added. A few of the return winners he smacked off Walts second serves -- and Connors got to look at a bunch of second serves -- punctuated that observation.

In the women's singles, 15-year-old Andrea Jaeger, the youngest player ever seeded at the Open, finally got to play her first match and thrashed Jeanne DuVall, 6-0, 6-2. Jaeger was solid as a wall in the backcourt, retrieving everything and pounding her shots with depth, pace and assurance.

Jaeger, who had a first-round bye, admitted that she had gotten impatient waiting to play. "School started Wednesday," said the sophomore at Adlai Stevenson High in Lincolnshire, Ill., "and here I am, sitting around, practicing. If I've got to miss school, I'd rather be playing matches."

Jaeger is seeded eight, but she is in the weaker half of the 96-woman draw, away from defending champion Tracy Austin and four-time champ Chris Evert Lloyd.

Jaeger was in the quarter diminished by the withdrawal of Wimbledon champion Evonne Goolagong Cawley -- her 18-year-old sister, Susy Jaeger was the "lucky loser" from qualifying who took Goolagong's place when she pulled out with a back injury, but was beaten today by Laura Dupont, 6-2, 6-1 -- so Andrea has a relatively comfortable draw to the semifinals.

"I'm not going to put extra pressure on myself by thinking about that," she said. Nevertheless, realists point out that Jaeger has an excellent chance of reaching the finals because Navratilova, the No. 2 seed, is playing shakily.

Navratilova doesn't like the high bounce of the courts, which reduces the effectiveness of her underspin approach shots and makes her hit high balls that put a strain on her suspect left shoulder. She would prefer a faster surface and lighter balls.

She is troubled more than most by the maddeningly high noise, confusion and aggravation levels of the Open, which has never coddled the players and subjects them to all the hassles of New York life. To aggravate matters, she suffers from allergies which flare up in the polluted Big Apple. m

Navratilova's usually powerful serve was, by her own admission, "very iffy." She was misjudging the range of her groundstrokes and bungling volleys she would normally make in her sleep. Her timing was off on both her footwork and strokes. But as poorly as she played, especially in the third set, Allen did worse.

"She wasn't getting many balls over the net, and then I got out of my rhythm. I'm just not playing well," said Navratilova. "I don't know what's the matter with me, but I just don't like this tournament . . . Other players feel the same way, but they don't want to be critical. I'm not one to mince words. I say what's on my mind."

Navratilova makes some valid complaints, but her babyish tone reduces their impact. However, the prize for whining today went to Nastase, hands down.

He moped and harangued his way through one of his most tiresome performances, which was mercifully speeded along -- Solomon was pummeling him. Later, he said he went on court angry because he had been promised a night match and then scheduled in the afternoon.

"I asked to play at night because I thought it would be cooler, and I'd have a better chance against Solly," said Nastase, a sad buffoon rather than a joyous clown prince today. "they said yes, but I guess when you ask for a favor in this tournament, that is the surest way not to get one . . . I was mad, because a promise should be a promise."

When Nastase lost his serve to trail, 2-4, in the first set, he kicked the ball into the stands. Then he began to stall, niggle, gesticulate, argue with umpires and linesmen, talk to spectators, curse and flip a familiar one-finger salute to hecklers in the stadium crowd. He tired quickly, and the more weary he got, the more baleful the soliloquies he addressed to his shoes and to the heavens.

Several times he seemed to erupt, and he was docked penalty points in the seventh game of the second set (a game point, since he was arguing a call that gave Solomon the advantage), and in the second game of the third set.

When he ran out of other nonsensical things to do, he ran off the court at 2-5 down in the last set and changed his shorts, trotting back a few moments later and presenting the sweat-soaked pair he took off to the umpire.

"I was just getting ready for my doubles -- they are so stupid here that they don't schedule me for anything yesterday, and make me play two matches one after the other today," Nasty said.

Solomon -- who has not lost a set since the second round of the Association of Tennis Professionals (ATP) championships, which he won last week in Cincinnati, just shook his head through all this.

"Obviously the point penalty system is in effect because those kind of antics aren't tolerated by anyone. We have a system for handling it now, and the system works. If you want to argue and scream obscenities and carry on, you're going to get penalty points or get defaulted. That's a decision you have to make on the court. If you want to act that way, you pay the price.