It was only after Pam Shriver had completed the painful business of slaughtering a wounded Lesley Hunt, 6-2, 6-0, and walked somewhat embarrassedly off the stadium court of the National Tennis Center yesterday that someone told her she is the youngest semifinalist ever in America's premier tennis championships.

Shriver celebrated her 16th birthday at Wimbledon July 4. She is 5 1/2 months younger than Chris Evert was when she reached the semifinals of the U.S Open for the first time in 1971. Ah, the little way that history teases us: Evert beat Australian Lesley Hunt too.

The late Maureen (Little Mo) Connolly was nearly 10 months older than Shriver when she first reached the semis of what was then called the U.S. Nationals in 1951. She went on to win the first of three consecutive titles, and remains the youngest champion.

"I don't realize it until somebody told me after the match," she Shriver, from Lutherville, Md., a bright and personable youngster who seems older than 16 because of her size (6 feet. 125 pounds) and her poise, on the court and in conversation.

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"I saw the special on Little Mo the other night," she continued, referring to a TV movie about the spectacular but tragic life of Maureen Connolly. "I wasn't sure if she was older or younger."

"Older in the semis, but still the youngest winner," she was told.

"So if I win this, I'd be the youngest champion?" Shriver tried out the notion, her brown eyes brightening. "OK, we'll have to work on that one."

It will be hard work, because her opponent today will be Wimbledon champion Martina Navratilova, whom she has played once before. She lost, 5-7, 6-2, 6-1, at Hilton Head Island, S.C., in April.

In the other semifinal, 1975-76-77 champion Evert will play Australian Wendy Turnbull, last year's runner-up. Turnbull, struggled with a shaky backhand and came within two points of defeat before elinimating Californian Kathy May yesterday, 3-6, 7-6, 6-3.

Turnbull, 25, was down 15-40 on her serve at 5-5 in the second set when she decided to stop trading ground strokes with the steadier May, and serve-volleyed her way out of trouble. She won the ensuing tie breaker, 7 points to 5, and pulled away from 3-3 in the final set.

Shriver's is an amateur, who cannot accept the $9,500 is prize money she has earned. She served well, getting 69 percent of her potent first serves in and scoring five aces. She hit some wonderful backhand approach shots, killer volleys and overheads.

But there was none of the exhilaration of her 6-2, 6-4 victory over No. 8 seed Kerry Reid in the fourth round, on an outside court. Shriver was troubled because she knew Hunt was nearly helpless. A doctor had advised her to default after her chronic had bad back spasmed severly Wednesday night.

The 28-year-old Hunt's mobility was limited, and she didn't serve hard and gave up the chase on many shots.

It is messy business, putting away an ailing but potentially still dangerous opponent, and Shriver did not like it. Neither did Hunt, who suffered more spasms in the dressing room after removing the lower back brace she wore during match. She crumpled on the floor and wept in pain, a towel over her head.

Shriver left the court unhappy, and broke into sobs on the shoulder of her coach of seven years, Australian-born Don Candy of Baltimore. He comforted her, ushering her out a back exit and away from a waiting pack of reporters, making sure she had a long shower and some lunch before dealing with her new celebrity.

"Pammy saw the doctor tape Lesley up and strap her into a corset before the match, and it upset her," Candy explained. "Some spectators around the court said "this Hunt can't play, she's not trying at all," and she overheard that. It got to the stage that the whole thing seemed hollow to her, and she just wanted to get off the court. She was embarrassed.

"She lost some composure, and at 3-0 in the second set she just wanted to serve as quickly as she could and get it over with. Here she was in the quarterfinals, and she felt awful. My comment to her was, 'You can only play who's out there.'"

"I was pretty uptight about this match - the quarterfinals, the stadium, the wind, Lesley's back," Shriver said. "I know I should win, but sometimes those are the hardest to win," continued Shriver, who missed the first day of classes at Baltimore's McDonogh School, where she is combining her junior and senior years, looking forward to early graduation.

Shriver was seeded No. 16, but not supposed to do better than her junior rival, 15-year-old Tracy Austin. Shirver never has beaten Austin in nine meetings but has now preceded her into the semis of a major tournament.

"Tracy started a year ahead of me," Shriver said. "This is her second year in women's tournaments. People compare us because of our ages, but I don't think about my age the way other people do. I just think of myself as somebody going to No. 1 eventually, no matter what my age is."

When she started beating established women last winter, way ahead of schedule, Shriver was deceived into thinking that it would be easy getting to the top. Three difficult weeks in England, culminating in the heart-breaking loss to Sue Barker at Wimbledon, convinced her otherwise.

"I realized then how tough it is mentally, and what it takes. Since then I've matured and started to know what it's going to take for me to be No. 1," she said. "I think that's why I've done well here, because I realized that."