Sometimes a tennis ball comes to rest atop the net and hangs there, stationary, for an instant. And sometimes, when it decides which way to fall, the match falls with it.

On the Centre Court at Wimbledon today, Pam Shriver, the Baltimore County 18-year-old who's always been as gifted as she was star-crossed, finally got the good roll.

A desperately tense net cord millisecond of truth went Shriver's way and, just minutes later, she completed a shocking victory over Tracy Austin, the world's second-ranked woman, 7-5, 6-4, to reach the semifinals of these championships.

"Now I know what the heck it all means. I can really taste it," said Shriver, a finalist at the U.S. Open in 1978, but, in the years since, the paradigm of a teen-ager assaulted by injury and disorientation. "This feels three times bigger than any victory of my life."

"Pammy picked a helluva match to get brave," said Don Candy, her lifelong coach. "She flew to the net on everything, even Austin's first serve. It was an all-out assault. I've never seen Austin nervous before."

This pair of 18-year-olds have battled since they were 11 years old. And Austin had won all 11 times they'd met. Until today.

Joining Shriver will be three ladies -- Chris Evert Lloyd, Hana Mandlikova and Martina Navratilova -- who, after their overshelming wins today, are obviously so close to the apex of their games that no one could sensibly handicap among them.

Evert, who will meet Shriver on Wednesday, and Mandlikova and Navratilova, one a Czech and the other a Czech expatriate, agree on one thing: all have never played better.

Mandlikova became the first player in 56 years of quarterfinal play to dismantle an opponent love-and-love with her 40-minute, 6-0, 6-0, humiliation of Wendy Turnbull.

Evert, who ran Yugoslavia's Mima Jausovec into a blond puddle of sweat, 6-2, 6-2, practically yawned, saying, "I haven't been tested yet."

What Evert had the good taste not to mention after her fifth straight-set victory here is that her half of the draw has worked out as luckily as John McEnroe's has among the men. It appeared that Evert would have to play both of those tormenting, exhausting baseline clones of herself -- Andrea Jaeger and Austin -- to reach the final. Now, both have been upset and Evert ended up matched with Jausovec, against whom she is now 13-0, and Shriver, whom she has beaten the seven times they have met. That's the rub of the grass.

Meanwhile, Navratilova just keeps rolling through the field, this time crunching Virginia Ruzici, 6-2, 6-3, in 55 minutes. So confident is the two-time Wimbledon champ that, just 45 minutes before her match, she ate a full lunch, consuming ham salad, potato salad, tomatos, peas, carrots and sundry victuals.

"Food never bothers me," said Navratilova. Neither does Ruzici, the Romanian whom she has now serve-and-volleyed into extinction 12 straight times.

If women's tennis has one glaring weakness, it is the paucity of genuinely compelling matches before the semifinals. The usual fare is just the sort of one-sided never-in-doubt destruction that Evert, Mandlikova and Navratilova wrecked. That wasn't the case this day. You can't have a more bated-breath, hang-in-the-balance match than Shriver's.

Which brings us back to that ball sitting on top of the net cord. It happened at 30-15 on Shriver's serve with the score, 7-5, 4-4, in the second set. To understand its impact, you have to know how Shriver and Austin got to the juncture.

Shriver, knowing that she had never won a first set from the implacable Austin, decided on new and daring tactics.

"The way Tracy serves, there are no surprises on either the first or second ball. So, Pam went to the net the whole time -- constant pressure," said Candy. "Once she got to the net, she hit her volleys right at Tracy, instead of away from her. That cut down the angles on the passing shots that Tracy hits best. And it meant she was getting a lot of bad bounces in the most chewed-up part of the court."

Shriver showed iron-nerved patience at the net as her strategy of blasting a first volley directly at Austin meant that she had to hit two, three or even four excellent volleys against Austin's whistline two-fisted passing shots before she finally could go for an outright angled winner.

Shriver also decided, in midmatch, to risk some tactics that, Candy said, "She'd done in practice but never taken out on the race course.

"Pam would charge to the net without ever making an approach shot. She'd just anticipate in the middle of a long baseline rally and, suddenly, in two jumps, she'd be at the net catching one of Austin's ground strokes -- even reasonably strong ones -- on the fly and volleying them for winners," said Candy. "I think it shook Austin. There was plenty going on in Pam's head today. She worked it out on her feet as the match progressed."

Even with all these surprises, Shriver needed every ounce of luck and guts. "If I ever played a perfect match, this was it," she said.

But it was a narrow thing.

On an Austin break point at 5-5, Austin trapped Shriver at the service line with a nasty half-lob. Retreating, Shriver leaped, and, with her back to the net, smashed a blind overhead backhand volley that spit-chalk on the baseline in the corner for an outright winner and a stirring Centre Court ovation.

Shriver served out that game victoriously, then set about finishing the set with a service break.

"I was lethargic. . . No, that's not the word . . ." said Austin, who gave one of the spunkiest, best-tempered press conferences any disappointed Wimbledon loser could ever muster. "Oh, what's the word?" laughed Austin, beginning to play charades with the world's press. "Sounds like," she said, pulling her ear. "Complacent. Yeah, that's it. I kept saying to myself, 'You can wake up and start playing any time now, Tracy.' "

Never fear -- you don't win $2 million by the time you graduate from high school, as Austin has, unless you can wake up when you need to. In the glorious second set, Austin broke Shriver's serve -- the heart of her game -- in the first, third and seventh games. If anything could unnerve the self-deprecating Shriver, that would do it. But Shriver surmounted her modesty and her frequent lack of self-confidence against the greatest players. Shriver answered back by breaking Austin's serve in the second, fourth and sixth games: with every minute of the 1-hour 47-minute battle, Shriver became bolder and less respectful.

Yet, even as Shriver blossomed, Austin slowly began finding the ground strokes, especially off the two-fisted backhand, that had been far below her standards all day. "I could feel myself getting back into the match," said Austin, who went from trailing, 4-2, in the second set to 4-4. "I was almost there."

Until the net cord shot. Leading, 30-15, on her serve in that ninth game of a deadlocked set, Shriver hit a swinging forehand volley that seemed to catch a full two inches of tape. But, somehow, the ball crawled up and over. "That's the point.I'll remember," said Austin, who never doubted that, if she could have gotten to 30-30, she could have broken Shriver's serve for the fourth time in the set.

The match finished on a third consecutive match point. Shriver volleyed, Austin retrieved, Shriver reflex-volleyed, and Austin cracked another ball back. The pair were only 10 feet apart.

And, one final time, Shriver's volley found a half-open court.

This wasn't the young, sometimes-nervous Shriver of 1978. This was a woman who has gone through two years of doubt about whether she would ever regain the glory she had as a 15-year-old. "I remember the moment it all changed," she said."Eleven o'clock in the morning, June 15th, 1979 at Chichester (England). Cold, windy, rainy, Friday. I hit my first warmup overhead and felt something pop in my shoulder. Next day, I couldn't lift my arm."

And couldn't for a long time afterward. "It taught me there's no clear sailing to the top," she said. "It's made me a tougher person. I've learned to play with my brain instead of my mouth," she said, even now an ice pack on her shoulder.

What will Shriver remember most from this victory? "The way I kept gambling, the way I kept saying to myself, 'Come in, come in.' "

This was the bright sunny day when Pam Shriver knocked hard on the doors of Wimbledon for the first time.

And the answer was clear:

Come in. Come in.