It's summertime, but the living's not easy at the U.S. Open tennis championships.

Oppressive mid-90 degree heat and humidity returned today, turning long matches on the asphalt-based hard courts at the National Tennis Center in Flushing Meadows into endurance tests. Harold Solomon's fitness saw him through an unexpectedly sticky five-setter against New Zealander Russell Simpson, but Wendy Turnbull and Virginia Wade lost limply; the meltdown of the Old Guard of women's tennis continued.

While John McEnroe was playing three impressive sets and one loose one in beating Rick Meyer in the 7,000-seat grandstand and Jimmy Connors was trouncing Terry Moor in the 19,000-seat stadium, Solomon was out on court 17, in the outback. He was there a long time, sweating and struggling with his own game before putting away the exhausted and cramping Simpson, 6-7, 6-3, 4-6, 6-0, Martina Navratilova, 15-year-old Andrea Jaeger and 18-year-old Hann Mandlikova -- the main contenders for the final in the botton half of the women's singles draw -- won their third-round matches in straight sets. But Turnbull, 27, and Wade, 35, went out looking Weak and Weary.

Turnbull -- runnerup to Chris Evert in the Open in 1977, the last year it was played on clay at nearby Forest Hills, a semifinalist the next when the tournament moved to Flushing Meadow; and the No. 5 seed this year -- was beaten by Californian Barbara Hallquist, 7-5, 6-1.

Turnbull, nicknamed "Rabbit" for her speed afoot, wasn't doing any hopping today. She has been suffering from flu and a sore throat and today suffered dizzy spells when she served. She wilted badly after having service breaks for 3-2, 4-3 and 5-4 leads in the first set.

"I was hitting pretty consistent service returns. That was the key today," said the 5-foot-9 Hallquist, a serve-volleyer and University of Southern California graduate who is No. 103 in the computerized world rankings. "She wasn't moving very well to the ball. She misjudged a few floaters and let them drop in for winners. This wasn't one of her better days, and I was able to catch her when I was playing well."

For a place in the quarterfinals, Hallquist will play Lucia Romanov, 21, the Romanian mechanical engineering student who eliminated the 12th-seeded Wade, 4-6, 6-2, 6-0.

Wade -- who won Americas premier tournament in 1968, the first year it was open to professionals as well as amateurs -- looked quite solid in taking a 4-0 lead in the first set, but infuriated Romanov and unwittingly turned the match around as she served for the set at 5-2, 40-30.

Wade hit her suddenly deteriorating forehand over the baseline on a long point, then argued that her serve had ticked the net cord; and managed to get the point played again. Romanov, incensed, promptly bashed three winners to break, and although she lost the set, was on her way.

The young Romanian, playing in the main draw of the Open for the first time after being runnerup in the junior event two years ago, started hitting out." At this point, all my tension left me. I was no longer nervous. All I could think of was that point. She was not very nice," Romanov said.

Jager, the youngest player ever seeded at the Open, fell behind Betsy Nagelsen 1-4, in the first set. Nagelsen had an advantage point for 4-2, but bungled two volleys and lost her serve as Jager lashed one of her two-fisted backhand down-the-line passing shots. From there, Jaeger accelerated to a 6-4, 6-2 victory.

Scrambling for some remarkable gets, her long blonde pigtails flying, Jaeger lobbed with uncanny accuracy and then found the groove on her passing shots. Twirling her racket between points, scampering with seemingly unlimited energy, Jaeger capitvated the stadium crowd, and a national television audience.

Her only problems were the 90-second changeovers, extended from the usual miniute to make time for TV commericals. She couldn't seem to play fast enough. "It takes a little while to get used to sitting down for that amount of time," said Jaeger. "I usually just get up and go. It's hard for me to sit still that long."

Mandlikova, the gifted Czech who beat Navratilova and Jaeger to win a pre-Open tournament in Mahwah, N.J., displayed her usual range of shots in beating Paula Smith, 6-2, 6-3, but got a scare as she served for the match.

Backpedaling to hit an overhead, she lost her footing, tumbled straight back, and banged her head on the court.

For a few alarming seconds, she appeared unconscious, but she got up and served out the last point. (Smith's high lob over her left shoulder had gone long, making the score 40-0).

Mandlikova next has a rematch with Navratilova, 6-4, 6-1 victor over Lindsay Morse today. Navratilova, the expatriate Czech who is awaiting U.S. citizenship, was the younger Czech's teenage idol. "She opened the door for us," said Mandlikova. "After she defected, the Czech federation realized we had to travel abroad to be good players. Now we have much freedom."

Connors, the three-time Open champ who has lost no sets and only 19 games in three matches, was in good form in routing Moor, 6-4, 6-1.

McEnroe, the defending champ and No. 2 seed behind Bjorn Borg, carved up fellow Long Islander Rick Meyer for two sets, hit a loose patch, then reasserted his serve and volley for 6-1, 6-1, 4-6, 6-2 victory.

Solomon admits that he went on court against Simpson, who is ranked only No. 144 on the computer, thinking ahead to his fourth-round match against Canadian Open champ Ivan Lendl, who had blown out 17-year-old Frenchman Thierry Tulasne, 6-2, 6-0, 6-1.

Solomon had thre set points on his serve in the first set then lost it in a tiebreaker. Simpson already looked exhausted, but dark clouds on the horizon indicated that a rain delay might be in the offing (as it turned out, thunderstorms did not hit until the match was over).

"I knew I should beat him, even though he beat in Tampa last April -- my first tournament back after a long layoff," said Solomon.

"But I kept thinking about other things and getting in trouble. It was strange. Now that (Vitas) Gerulaitis lost early, I'll move past him to No. 5 in the rankings. That was the goal I set 15 months ago, to crack into the top 5, and now that I've done it, I was thinking about it out on the court.

"These are the toughest kind of matches sometimes -- when they stick you on Court 17, back in the boondocks, against a guy you're supposed to beat. You go out there thinking ahead to the next round, a big match, and kind of just want to get this one out of the way. The next thing you know, you're hitting tentatively, balls are going short, the guy is coming in on you, and you're into a dog-eat-dog match," added the No. 7 seed.

When Solomon trailed, two sets to one, Court 17 claustrophobia corner. As many of the 19,977 afternoon spectators as could get a vantage point crammed around the court for a view of the possible upset.

But Solomon never thought he would lose. He is in good shape after winning the Association of Tennis Professionals Championship in Cincinnati a week ago, and could see that the 6-2, 180-pound Simpson was tired, tightening, and fading fast. "He was completely wasted," said Solomon, who ran through the last two sets, barely pausing to change sweat-soaked shirts a couple of times.

In the featured night match, Brian Gottfried defeated Ross Case, 6-0, 6-7, 6-4.

In other matches, Frenchman Pascal Portes upset Victor Amaya, 6-3, 2-6, 6-4, 6-3; Elliot Teltscher ousted Italian Gianni Ocleppo, 7-5, 6-0, 6-2; Bernie, Mitton beat 17-year-old Sammy Giammalva Jr. 6-3, 2-6, 6-2, 6-3; and Johan, Kriedk advanced by default over Tony Giammlva, 22, the other touring son of former pro and Houston coach Sammy Giammalva, Sr., who suffered an allergic reaction to medication he had taken to combat cramps.