Martina Navratilova, the reigning Wimbledon champion, prevailed today with new rackets she had hurriedly flown in from Dallas after a New York cabbie drove off with her old ones while she shopped for groceries Friday afternoon.

Tracy Austin, the 16-year-old high school sophomore and suddenly wealthy tennis prodigy from California, needed a bold serve and a desperate, take-a-guess-and-stab volley to survive set points and grab control of her semifinal match at Madison Square Garden.

Chris Evert, still distraught after back-to-back defeats that eliminated her before the semifinals, flew home to Fort Lauderdale to make final preparations for her April 17 wedding to Englishman John Lloyd, which is where her mind has been all week.

Welcome to "As the Fuzzy Ball Turns," also known as the $275,000 Avon Championships, first jewel in the modern "Rriple Crown" of women's tennis.

With Evert missing the semifinals because of back-to-back losses to Austin and Dianne Fromholtz in the double-elimination portion of the tournament, Navratilova and Austin became the odds-on favorites to rule this eight-woman playoff, which climaxes the $1.9 million women's indoor circuit taken over this year by Avon from former sponsor Virginia Slims.

The obvious heirs to the throne abdiacated at least temporarily by bride-to-be Evert, they both reached Sunday's title showdown (WRC-TV-4 at 2 p.m.) by winning three straight matches without a loss.

In today's semifinals, Austin recovered from a shaky start and a 2-5 deficit in the first set, saved those two set points on her serve at 4-5, and then accelerated past Fromholtz, 7-5, 6-2.

Navratilova, the 22-year-old expatriate Czech left-handler, then overwhelmed Englishwoman Sue Barker, 6-1, 6-3, in a 50-minute blowout that left 12,153 spectators marveling at Navratilova's power and yawning at the ease with which she prevailed.

Navratilova has beaten Austin three times since losing to her in the final of the circuit's inaugural tournament in Washington in January. She will be favored to capture the $1000,000 top prize, and relegate Austin-who has earned more than $120,000 in prize money since turning pro last October-to the $52,000 runner-up prize.

But both players are sharp and eager, and should provide a finale of faseinating contrasts. Naratilova likes to serve and volley, attacking all the time, while Austin-often likened to an Evert clone-prefers to play a more defensive back-court game.

Austin started fitfully today. Normally a sprite on the court, she ran hesitantly, sluggishly, at the outset, and didn't find the range on her ground strokes until she was 2-5 down in the first set. But when she held serve for 3-5, little smiles of confidence started to replace dispirited frowns in her determined teen-age countenance.

Fromholtz rushed herself as she served for the set-"She looks like a little windup toy," commented one courtside onlooker - and promptly lost her serve at love with a sloppy game.

Austin, still struggling, overhit first a forehand and then a backhand a fall behind, 0-30, and twice faced set points on advantages.

On the first, she surprised Fromholtz with a good, deep first serve wide to the Aussie's left-handed fore-hand. Formholtz, 22, who won the tournament in Boston last week and got into the semis here by knocking out Evert on Friday, hit her return weakly into the net.

On the second set point, Fromholtz jerked Austin into the forecourt with a backhand drop shot, and wound up to rip her favorite passing shot, a big backhand cross-court, off Austin's covering shot to the middle of the court.

Little Tracy was a sitting duck at the net, but she anticipated Fromholtz's top-spin screamer, lumged, and made a forehand stab volley that just greased over the net and died for a winner.

Fromholtz lost her serve at 30 in the next game. Austin held at love for the set, and soared from there. The bounce was back in her legs, the crispness in her ground strokes, as she broke to a 4-1 lead and closed out the 82-minute match.

Navratilova had problems whatsoever with Parker, who was a bit stifflegged after suffering cramps in a doubles match that did not end until 12:35 a.m. this morning. (Barking and Ann Kiyomura beat Evert and Rosie Casals in Friday's late semifinals, 6-4, 2-6, 6-2, but lost to Betty Stove and Francoise Durr, 3-6. 7-6, in today's doubles final.)

Navratilova, hitting clusters of winners from the start, sprinted to a 3-0 lead in the first set, played one careless game to lose her serve at love, and them simply overpowered the 5-5, 115-pound Barker, physically and psychologically.

Navratilova has not lost a set here, but encountered her greatest opposition from the cab drive who was transporting her home from practice Friday. At 4:15 pm., she stopped for groceries, left her rackets in the car, and asked the man to wait with the meter running. She was trying to get home in time to receive a 4:30 call from Czechoslovakis.

When she came out of the store, the taxi was gone. "I guess he figured the rackets were worth more than a $3 fare," she said. "I also missed my call."

At an earlier stage of her career, such misfortune would have sent Navratilova into a self-destructive snit. But she calmly called her secreatry in Dallas, her adopted hometown, and had a new set of rackets put on the first plane to New York. They arrived Friday evening.

"I'm playing the best I ever have, I'm just hitting the ball with more pace, my backhand has improved tremendously, I'm moving better, and my nerves are even more under control than they were last year," said Navratilova, who has her 84-year-old paternal grandmother, Audela Subertova, here with her.

No one was silly enough to disagree. CAPTION: Picture 1, Chris Evert finds herself in an unusual role after suffering consecutive losses to Tracy Austin and Dianne Fromholtz, UPI; Picture 2, in Avon championships and being ousted from the meet. UPI