Some top seeds wither away in the torrid US Open

Approximately midway through the US Open Tennis Championships, slightly more than half the seeded players have already been checked out of the competition. None, it should be added, were seen wearing "I Love New York" buttons as they exited the locker room.

The city's been steaming, making play on the ovenlike courts of the National Tennis Cenendurance and fitness.

The big names that have fallen, of course, haven't wilted in the heat so much as they've fallen prey to "hotter" opponents.

Take Martina Navratilova, for example. Though second-seeded among the women, she ran into a rapidly blossoming star, Hana Mandlikova, and fell to the 18 -year-old Czech 7-6, 6-4 in a fourth-round match. Mandlikova is no unknown to those who keep abreast of the game, a fact indicated by her No. 9 seeding here. In the semifinals of the New Jersey warm-up tournament preceding the Open, she had beaten Navratilova, a Czech defector who soon becomes a US citizen, then gone on to knock off American teen-age whiz Andrea Jaeger in the final.

Among the men, No. 4 Guillermo Vilas and No. 5 Vitas Gerulaitis have been the highest seeds sent packing. Vitas, a finalist last year, was ambushed by bullet-serving Hank Pfister in a second-round thriller, which saw the unseeded Californian win in a fifth set tiebreaker, 6-3, 6-2, 1-6, 3-6, 7-6 (8 points to 6). The match was played at night, when anyone who serves up smoke is dangerous.

Vilas's departure from the tournament was less shocking, since his conqueror, Wojtek Fibak of Poland, is a gifted player whose game is perhaps better suited to the Open's asphalt courts.

Vilas, for the many who may have forgotten, won the tournament in 1977, the last year it was played on a slow, claylike surface and the last US Open at Forest Hills.

Since then, physical difficulties have hindered the Argentine left-hander, who had begun to feel comfortable playing nonstadium matches. "My first match was on court No 3, my next on No. 16. I was thinking maybe my next should be in Manhattan," he joked. Instead, he was thrown into the stadium court against Francisco Gonzalez, who extended him to five sets, before losing to Fibak 3-6, 6 -3, 6-4, 6-3 on the grandstand court.

After his troubles against Gonzalez, Vilas explained by the lesser-knowns become wolves in sheep's clothing at this end-of-summer chanpionship. "I'm all the time worried about these guys that have nothing to lose," he said. "They have a goal, to be [ranked] in the top 30 or 40 in the world. For them, the summer is over then they reach this goal."

The player with the most at stake, though he won't admit it, is Bjorn Borg. The US Open title has eluded the five-time Wimbledon champion, who needs to win here to keep his Gran Slam hopes alive. He's halfway "home" with the French and Wimbledon US victory and then one in Australia to become the first slam winner since Rod Laver in 1969.

Borg has advanced to the quarterfinals, where he meets a slimmed-down Roscoe Tanner a year after losing to him under the lights at the same juncture in the ' 79 Open. To Borg's relief, the rematch will be played in the afternoon, when he will have less difficulty following Tanners' screaming, left- handed serves.

Though Borg has looked less than untouchable, dropping sets to John Sadri and Peter McNamara in his first four matches, he hasn't been favoring he leg that caused his default in the Canadian Open two weeks ago.

John McEnroe, the defending champion, actually figures the resurfaced courts could aid the Swede. "It seems like them made these courts for Borg, not the Americans," he said. "The surface is slow and produces high bounces. I would like them faster."

Jimmy Connors, the only player to win the Open on three different surfaces, can't complain. He sliced up his first three opponents -- Marcel Freeman, Butch Walts, and Terry Moor -- without losing a set.

Chris Evert Lloyd has been just as impressive in the womens's singles, whosing she means business in dropping just nine games in her first four matches. Chris's four-year US reign was disrupted by Tracy Austin last September, but her game looks to be as good as ever, good enough to regain the title and secure the No. 1 world ranking. Evonne Goolagong, who beat her in the Wimbldeon final, isn't around, having withdrawn at the last minute with an injury. (Billie Jean King also passed up playing in the singles.)

Assuming Evert Lloyd beats Mimi Jausovec in her next match, she then will meet the winner of the Austin-Pam Shriver showdown in the semifinals. Tracy, like Chris, plays a baseline game that is characterized by maddening consistency and accuracy. Shriver, on the other hand, can serve and volley with anybody, using her long reach and oversize racket to maintain reletnless pressure at the net. Two years ago she became, at 16, the Open's younges finalist, but has had to come bach from a shoulder injury since then.

It's quite likely that either Mandlikova or 15-year-old Andrea Jaeger will emerge from the other half of the draw to reach the final. Jaeger's style resembles that of Evert Lloyd and Austin, while Mandlikova's fluid all-court game is often compared to Goolagong's.