Kathy is all giggles -- until she crosses rackets with the older pros

The adolescent giggles of Kathy Rinaldi belie the fact that this exuberant 14 -year-old is a successful professional athlete.

These high-pitched laughs - as characterisic as the ever-present reporters who have trailed the Florida teen-ager since July, when she became the youngest tennis player ever to turn professional - disappear quickly, however, when she takes to the court.

''I just go out there and play to my ability and don't think about my age,'' said Kathy, who has catapulted from No. 181 on the Women's Tennis Association's (WTA) rankings to No. 25 in the last 10 months. ''I should get stronger. I hope that comes with age, but I guess I'll just have to wait and see. I'm very happy with my decision to turn pro, because I'm really enjoying myself.''

Kathy particularly enjoyed her October trip to Japan, where she captured her first tournament crown as a professional, the Borden Classic in Kyoto.

''The first victory is always great,'' said the 5 ft., 5 in., 110-pound ninth-grader, who defeated 19-year-old Julie Harrington of Spokane, Wash., in the championship match by a score of 6-1, 7-5. ''The matches were tough, but I just played my game. And Japan was fun. The people are so nice.''

Kathy is accustomed to firsts. She stunned the tennis world at the age of 12 by winning all four major US titles in the girls' 12-and-under division - the hard courts, clay courts, indoors, and nationals - an unprecedented United States Girls' Grand Slam.

Her appearance in May's French Open made her the youngest player to enter that prestigious event, and she reached the quarterfinal with upsets over eighth-seeded Dianne Fromholtz and 11th-seeded Anne Smith. A month later the Wunderkind became the youngest player to win a match in the 104-year history of Wimbledon when she defeated Sue Rollinson of South Africa, 6-3, 2-6, 9-7.

These surprising accomplishments helped hasten the decision to turn pro on July 24. The pony-tailed Floridian was 14 years and four months old on that date , beating Andrea Jaeger as the game's youngest pro in history by four months. In perspective, Tracy Austin turned pro at 15 years, 10 months, and Chris Evert Lloyd, Kathy's idol, waited until the ripe old age of 18.

''My family and I reached the decision together,'' the Martin County High School student said. ''We decided that at this stage of my career it is best to play a consistent WTA tournament schedule. By playing in a select number of events, I am still able to devote a lot of time to my schoolwork.''

Kathy's parents, Dennis and Lindi, were naturally apprehensive about their daughter's joining the professional tour and have attempted to protect her from the spotlight and preserve her private life.

Meeting Kathy convinces you that this precarious balancing act has been a success to date. Although she takes her sport seriously, she retains the endearing personality of an unaffected teen-ager. She loves ice cream and pizza and bowling or playing softball with friends. But not surprisingly, she also enjoys the travel, sightseeing, and competition of the professional tour.

Her father, a Jensen Beach dentist, said that the family actually delayed Kathy's professional debut and that her schedule as a junior player - competing in both amateur and professional events - was at least as busy as her current slate.

''When I go on trips I get homework and do it on the road,'' Kathy said. ''It's sometimes hard to sit down and do it, but I like all my subjects and am enjoying high school.''

The right-hander's tennis ability never was a question, and her performance in her short professional career has shown why. Kathy possesses a sound baseline game, quick footwork, intuitive strategy, a strong two-handed backhand, and an intense concentration that has thwarted many tour veterans.

Life on the road is trying for any athlete, but Kathy has made a lot of friends on the tour, including her doubles partner, 18-year-old Pam Casale of Fairfield, N.J., who turned professional five months before Kathy. Kathy had to face her best friend and partner in the semifinal at Kyoto.

''On the court we're competitive, and I just have to block it out of my mind that we're friends,'' said Kathy, who rallied for a 3-6, 6-3, 6-1 victory. ''Off the court we're good friends, and that's why we decided to be doubles partners. We thought it would be fun.''

Kathy is the youngest member of a very tennis-oriented family, and in fact got her first exposure to the game when she started tagging along to the courts with her mother at the age of 4. Her father, who took up the sport at about that same time, is now a good club player as well as Kathy's current coach and trainer. Kathy's older brother Denny went to college on a tennis scholarship and now is a dental student at the University of Florida. Tina, her older sister, was the No. 1 singles player at Martin County High and is now on the University of Virginia tennis team. But brother Bill, a freshman at Florida, has currently forsaken the tennis racket for baseball.