Hana Mandikova of Czechoslovakia upset expatriate Czech Martina Navratilova, her one-time idol, for the second time in nine days, Pam Shriver cleverly blended patience with her customary aggression to oust Dianne Fromholtz and Tracy Austin ran off 10 straight games from 2-4 down to rout Virginia Ruzici today, assuring that there will be at least three teen-agers in the women's singles quartefinals of the U.S. Open.

Fifteen-year-old Andrea Jaeger is favored to beat Renata Tomanova, and 19-year-old Ivanna Madruga is a good bet against Candy Reynolds on Tuesday, which would mean that five of the last eight women in America's premier tennis tournament are under 20 years of age.

Perhaps the most naturally talented of the teens who are taking over women's tennis is Mandlikova, 18, who beat 1978/79 Wimbledon champion Navratilova for the first time a week ago Saturday in the semifinals of a tournament of Mahwah, N.J. She repeated the triumph today, 7-6, 6-4, to the delight of a howling, near-capacity crowd in the 19,000-seat stadium of the National Tennis Center.

Shriver, 18, who two years ago was the youngest finalist in the tournament's history and last year was an injured, depresed, first-round loser, kept her cool after losing a tough, hotly contested first set in the muggy late afternoon heat and beat No. 7 seed Fromholtz, 3-6, 6-1, 6-1.

Austin, 17, who last year dethroned four-time champion Chris Evert Lloyd in the final and became the youngest champion, fiddled for six games, then burned Ruzici with a stream of hard, deep, unerring groundstrokes, 6-4, 6-0, impressing the evening crowd of 8,860.

Earlier, in the heat of the day, Evert continued her relentless march to the quarterfinals by beating JoAnne Russell -- an affable fellow Floridian who says she now plays out of Runyon's,an East Side Manhattan saloon that is popular with the sporting set -- 6-2, 6-1.

Mima Jausovec, a semifinalist in 1977, the last year the Open was played on clay at nearby Forest Hills, upset 20-year-old Kathy Jordan, 7-5, 6-3. The rest of the women's fourth round will be played Tuesday.

The most dramatic victory belonged toMandlikova, who beat the woman whose game she most wanted to emulate when she was a junior in Czechoslovakia, and whose defection to the United States during the 1975 Open, she says, "opened the door to other Czech players to travel without restrictions, and to keep their prize money."

Mandilkova, daughter of a two-time Olympic sprinter, is a natural athlete who hits a bewildering range of shots with seemingly effortless fluidity. She beat Navratilova in three sets on a similar surface (cushioned hard courts) at Mahwah, but wasn't sure she could doit again in a big tournament -- even though she is confident now under the tutelage of Dutch player Betty Stove.

("She is a very good tactician," Mandlikova said, "I believe in her.")

"I was sure it would be much harder this week, with everybody looking," said Mandlikova, who beat Jaeger in the final of the pre-Open tournament, and is rapidly shedding a reputation as a brilliant one-set player who-cannot close out a match. "I needed everybodylooking. I wanted everybody looking." c

They played in the still brutal heat and humidity of late afternoon. Mandlikova wore an orange skirt, and a scarlet headband encased her long, brown hair. After a few games, her complexion matched the garments.

Her game was also vivid colors. Shehas remarkable racket control and command of spin, and she threw a little of everything at Navratilova. She served and volleyed. She stayed back. She lobbed, drop shotted, dinked and used the full repertoire of shots. Most effectively, she took advantage of Navratilova's errant serve, attacking her second serves, getting to the net for sharply angled volleys.

"Mandlikova's maturing on the court and, boy, when she plays well throughout a match, shes like the best player in the world," Shriver said.

"She likes to hurt the ball, like me,but whereas my game is 90 percent serve-and-volley, she can fiddle around,make drop shots, running lobs, running cross-court topspin God knows what. It's almost scary how much she can do with the ball."

Shriver did much more than just serve-and-volley today -- she moved the ball around smartly, railled until she got her openings, then came to the net for killers -- but her point about Mandlikova was well taken.

After an early exchange of breaks, the first set was decided in a best-of-12-point tie breaker that Mandikova won, 7 points to 2. She played it almost flawlessly, winning all five points on her serve, chipping wonderful returns of second serves and gliding to the net to take command of the first two points that Navratilova served.

Mandlikova broke in the first game ofthe second set, Navratilova netting one of many forehand volleys, and led, 3-1. Navratilova -- who hates the Open and never has played well here, losing to Shriver, then Austin in the semifinals the past two years -- seemed on the verge of unraveling totally, then broke back to 3-3 after Mandlikovahad a point for 4-1.

But at 30-40 in the next game, Mandlikova chipped an angled backhand cross-court return that looked wide but was called good Break. Navratilova grabbed her head, pointed to the line, screamed, and went to the umpire's chair, gesturing with two fingers an inch apart, to show how far the ball had been wide. She probably was right, buthas cried "wolf" too often in the past on line calls. The decision stood.

Mandlikova held for 5-3. Navratilova bravely saved a match point and held for 4-5, then got to 15-40 as Mandlikova served for the match.

Mandlikova saved the two break pointswith a great backhand volley and a good serve wide to the backhand. The Navratilova hit a forehand pass long and a backhand return into the net to end it. Mandlikova jubilantly thrust her arms high into the air and ran to a courtside box to embrace friends. She was mobbed by well-wishers and autographs-seekers.

Mandlikova said she did not call hometo Prague to tell her parents that she had beaten Navratilova the first time until five days after the fact. They already had read about it in the newspaper. "They told me I must try to keep my head on the floor," she said,"to stay normal."

She is not getting big-headed, but she knows that instead of being a few points away from beating the top players, she can now win.

"I'm there. I really think so. I think the big points are mine now," she said."It's true. I needed this. Now I have very much confidence. I feel I can beat anyone."

Shriver, the 6-foot Great Whooping Crane from Lutherville, Md., is perhaps where Mandlikova was a few months ago. She hasn't lost to anyone ranked below her (she is No. 14 now) since February, and has played well against the top players but just hasn't beaten them.She feels her breakthrough is imminent, too -- and hopes it comes against Austin in the quarterfinals here.

"If really think if I play well, I could win the tournament," she said."I think this is the most open big tournament that I've ever played in. There are six or eight people who couldget to the final, or even win. It's going to be interesting."