In perhaps the most startling women's singles upset ever in a major championship, 16-year-old Pam Shriver defeated Wimbledon champion Martina Navratilova yesterday, 7-6, 7-6, to become the youngest finalist in the history of the U.S. Championships.

Shriver's coach, Don Candy, said Thursday the semifinal match against Navratilova, the top seed here and dominant player in women's tennis this year, would be a picnic for his protege.

"If she wins, everybody will say, 'Great, terrific' If she loses, it's 'Oh well, what do you expect?A kid against the No. 1 girl in the world.' Pammy can't lose either way. All the pressure is on the other girl," said the coach.

Shriver, the 6-foot and toweringly poised high school senior from the Baltimore suburb of Lutherville, Md. - who turned 16 on July 4 - took his words to heart and had a picnic on a drizzly day.

The main course was Martina Navratilova.

An amateur who does not plan to turn professional until she graduates a year early from McDonogh School in Baltimore in June, Shriver cannot accept either the $38,000 first prize or $19,000 runner-up money.

But there will be a handsome silver cup for her if she can win tomorrow's final against either 1975-76-77 champion Chris Evert or last year's runner-up, Wendy Turnbull of Australia. Evert led, 3-1, in the first set when their semifinal match was suspended by rain yesterday. It will be resumed at that point today, men's semifinal day.

Shriver, who also is in the semifinals of the women's doubles with Floridian Betsy Nagelsen, was seeded No. 16 on the basis of her performance in her first nine months of women's tournaments. She never had played in the Open before, never entered a pro tournament before the Virginia Slims of Washington in January.

Considering that, her impressive, neverless triumph yesterday was more stunning than Evonne Goolagong's victories over Bille Jean King and Margaret Court to win Wimbledon in 1971; Kerry Melville's triumph over Wimbledon champion King in the second round of the U.S. Championships in 1966, and the promising but then unproven Billie Jean Moffitt's second-round win over Margaret Smith (later Court) at Wimbledon in 1962.

If there have been more astounding results among the women in the Wimbledon and U.S. championships, aficionados with long memories could not recall them yesterday. Shriver's net-rushing afternoon at the new National Tennis Center in Flushing Meandow Park may have been The Ultimate Picnic.

"The only time Pammy will feel pressure is if she happens to break serve and lead, say, 4-2, 30-15 on her serve. Suddenly she'll realize she has a chance to win the set, and then we'll see what happens," Candy had said.

Shriver broke Navratilova's powerful serve twice yesterday - in the first game of the match and the fourth game of the second set. Each time she lost her own serve, double-faulting twice, in the very next game.

But other then that, Shriver played inspired tennis - serving and volleying superbly, hitting out in situations where it would have been easy to clutch, spinning and loping on her long slender legs for "gets" of shots that seemed beyond her determined reach.

It would be difficult to imagine a match with more nerve-wracking distractions.

The second set was interrupted twice by rain - for 48 minutes with Navratilova serving at 1-2, 15-0, and for 24 minutes more with Shriver serving at 3-3, 30-0.

The leaden skies caused an ajustment in aircraft approach patterns to nearby LaGuardia Airport, so after the second delay, planes were roaring overhead frequently, deafeningly, seemingly low enough to reach out and touch.

Spectators in the crowd at Louis Armstrong Stadium - announced at 16,251 paid, but with many no-shows - kept seeking cover from the drizzle, then wandering back to their seats during play.

Through this all, Shriver kept a sublime cool, saving two set points on her serve at 4-5 in the first set and two more at 5-6. She then played a best-of-12-point tie breaker nearly flawlessly, winning it, 7 points to 3.

In the second set Navratilova, briefly digging down and finding the reserve of guts that allowed her to come back from 2-4 in the third set of the Wimbledon final against Evert, fought off three match points from 4-5, 15-40.

But Shriver again was a killer in the tie breaker, missing only one first serve and seizing it, 7-3, with a "damn the torpedoes" backhand approach shot that forced Navratilova to overhit a forehand on the fifth match point.

"Even though it got tight, I seemed to take my time and handle the pressure pretty well, better than I thought I might," said Shriver, who is 10 months younger than Maureen Connolly was when she reached her first final and won the first of three consecutive titles in 1951.

Navratilova, an elderly 21, did not regard Shriver as a tender youth, or herself as a potential child molestor.

"I look across the net and up at her," said the expatriate Czech, who at 5-feet 9 and 135 pounds is three inches shorter and 15 pounds heavier than Shriver. "Age doesn't really have anything to do with it. She played like a grown-up, a 25-year-old. You think, I'm playing a good player,' not 'God, she's only 16.'"

Shriver served oppressively, especially on the most crucial points, missing only one first serve on the set points against her and one in each tie breaker.

She mixed up the pace and spin smartly, but relied heavily on a wide slice to Navratilova's left-handed backhand, which bounced high on the rubberized asphalt surface, giving her a chance to get to net behind it and handle the returns - when they came at all - with agile volleys.

That is a difficult serve to handle under any circumstances, but more so because Navratilova has tendonitis in her left shoulder, and took heat treatments to try to keep it loose during the two rain delays. She was not able to get her usual penetration on her backhand, especially on high balls.

"I usually serve well against left-handers, kicking it away from them on the backhand," said Shriver, who started a bit shakily but then was "in the zone", as tennis players call the dreamlike surge when every reflex is correct and every critical shot stays in the court. "I don't think I can serve much better than I did today."

WOMEN's SINGLES

Semifinals

Pam Shriver def. Martina Navratilova, 7-6, 7-6.

MEN'S 35 SINGLES

Final

Marty Riessen def. Jim Parker, 6-2, 6-1.