For anyone who might have missed the trend, a new generation has taken over women's tennis, as was unmistakably demonstrated on the stadium court of the National Tennis Center today when 17-year-old Tracy Austin wiped out 31-year-old fellow Californian Rosemary Casals, 6-0, 6-0, in the second round of the U.S. Open.

There was a message to this brutal mugging in broad, 116-degree daylight. Casals, well-known for her feminist persuasions ever since she crowded shrilly on television the night Billie Jean King, her comrade, knocked the oink out of male chauvinist pig Bobby Riggs, smilingly refers to herself as one of tennis' "old broads." The changing of the guard could not have been more convincingly symbolized had a troupe of scarlet-coated Grenadiers marched in front of the court.

Austin is the defending Open champion.Last year, she ended Chris Evert Lloyd's reign one match short of an unprecedented fifth successive title and became -- at 16 years, nine months -- the youngest champion in the history of America's premier tennis tournament.

Austin is the child queen, but she cannot count on ruling for as long as some of her predecessors. A whole group of ascending princesses, after a couple of years in anxious waiting, are ready to usurp the throne. In the process, they are expelling from the court of honor the generation of pioneers Casals and King.

The major titles in women's tennis in the 1970s belonged almost exclusively to six individuals: King, Margaret Court, Evert, Evonne Goolagong, Martina Navratilova and Virginia Wade, who intruded for a sentimental triumph in the centennial Wimbledon of 1977.

King and Court gave way to Evert and then Navratilova at the top, but for a decade one could pretty much count on form holding true in women's tournaments, and the same players parading to the quarterfinals week after week.

There was a solid hierarchy of players who seldom won titles, but were regularly in the quarters and semis, occasionally the finals. Casals epitomized this group, which also included Betty Stove, Kerry Melville Reid, Wade and Francoise Durr. In recent years, they were gradually replaced by Wendy Turnbull, Dianne Fromholtz, Greer Stevens, Virginia Ruzici.

The old generation is pretty much phasing out now, although matriarch King -- out of the Open singles because of a persistent virus -- is still the leader of that band, ranked No. 5 in the world at age 36.

Meanwhile, the middle generation -- the Turnbulls and Fromholtzes and Stevenses -- are being replaced by a bumper crop of young players who appear perfectly capable not only of reaching quarterfinals, but of winning tournaments. Foremost among them are Austin; Andrea Jaeger, 15, who last week became the first player younger than Austin to beat her; Hana Mandlikova, 18, who beat Navratilova and then Jaeger to win a New Jersey tournament last week, and Pam Shriver of Lutherville, Md., the 1978 Open runner-up who has made a full-scale comeback at age 18.

Right behind them, gifted, but as yet uncertified as championship material, are the likes of Kathy Jordan, 20; Ivanna Madruga, 19; Sylvia Hanika, 20, and Bettina Bunge, 17.

They have been creeping up, and appear ready to dislodge King, No. 6 Turnbull, No. 7 Fromholtz and No. 8 Stevens from the world top 10, perhaps this autumn. Jaeger is already No. 9, the youngest player ever to crack the top 10 and winner of nearly $100,000 in eight months as a pro. Mandlikova is No. 10, and talented enough to be No. 1 some day if she can play as she did last week in Mahwah, N.J.

"I saw Hana play Martina, and when she won the match, I thought that could be a turning point, because she finally got over that barrier of being ahead and then choking," said Shriver, who today crushed Renee Blount, 6-1, 6-2, and looks to be in impressive form. "That could mean something to her mentally, especially since she went on to beat Jaeger and win the whole tournament."

Of the old guard, Stove and Stevens lost in the first round of the Open; Reid is at home in Australia expecting a baby; and Durr has retired to over-35 competition. Wade barely escaped 20-year-old Argentine Claudia Casabianca today, 4-6, 6-3, 7-5. Goolagong, who last month won a Wimbledon title almost as sentimental as Wade's, is out with a bad back. Evert is expected to reach a semifinal showdown with Austin here, but like Goolagong, has made it clear she won't play too many more summers.

"The 30- to 35-year olds are kind of fading away," said Shriver. "I think there's going to be a big turnover, if you measure it from last year to next year, and that should be good for women's tennis. New faces are really nice," said Shriver, who has put her aggressive game back together in the past nine months after a shoulder injury and a series of psyche-bruising losses jarred her from her autumn 1978 cloud to a 1979 of lows that bordered on depression.

Now ranked 14th in the world, she has not lost to a player ranked beneath her since February. After two easy rounds here, she next meets Bunge -- "a better measuring stick of how I'm playing." Looking ahead in the draw without taking anything for granted, Shriver expects to beat Bunge and the struggling Fromholtz to reach a quarterfinal meeting with Austin.

"It's kind of funny, but I can say that finally I've started to feel comfortable in early round matches," said Shriver, who lost only a handful of points on her serve today.

"Nobody can tell you when or how you get this feeling, but it just all of a sudden comes, where you go out on the court and you just know you're going to win. It's just a question of doing the things you know how to do best. Whereas a year or even six months ago, I'd go out against somebody I knew wasn't as good as I am and wasn't sure I could beat them. It's just a matter of mental confidence, and I've really come on the last few months."

So have her contemporaries. The future clearly belongs to Austin, Jaeger, Mandlikova, Shriver, et al -- and, at least for a while, to Navratilova, who admits to feeling ancient at 23.

"The older players are getting older and worse, and the younger players are getting older and better," she said Wednesday night after straining to an uneasy, 6-4, 4-6, 6-2 victory over 19-year-old Lena Sandin, whom she had never seen or heard of before last week. "At this point, I'm one of the older players. The young ones seem to be getting better at an earlier age. Jaeger is better than Austin was at 15. The trend may just keep going."

A couple of years ago, it would have been unthinkable for a leading lady to have the sort of trouble Navratilova did with an anonymous player like Sandin, ranked only No. 65, in the first round of a major tournament.

"I remember having arguments with Billie Jean King 10 years ago about the depth in women's tennis. I don't care what anyone says; there was none," recalled former Australian Davis Cup player Owen Davidson, now Jaeger's coach.

"Now, it's a lot different. There are good matches in the second and third rounds. And I think 1981 will be a great year, because there are a whole bunch of kids -- Austin, Jaeger, Mandlikova, Shriver, Bunge -- who have what it takes.The players who don't have it will fall away pretty quickly in the future, not hang around the edges like in the past."

Casals and King used to say there was depth in women's tennis -- before it was true -- in a well-intentioned effort to hype the gate at their tournaments. They know the truth in Davidson's words.

King, apprised of Davidson's remarks, chuckled and said she remembered their friendly arguments differently. "I agreed with him that we didn't have depth," King said. "I said it would take 10 years, and it's been 10 years. Now age has taken care of a few of us -- Betty and Rosie and Virginia and Kerry and myself. The new kids are better, and the evolutionary process is speeding up. Ther is a new generation. The guard has changed. That's inevitable, and it's good for the game."