A year ago, Martina Navratilova is the first to admit, the prospect of playing 15-year-old prodigy Tracy Austin in the fourth round at Wimbledon would have done strange things to her mind. Her nervousness would have eaten at her, and probably manifest itself in temperamental outburst on the court.

But at age 21, Navratilova has matured. The experiate Czec left-hander, now living in Dallas, has gained self-control to go with the aggressive game that is so effective on the grass courts here.

She calmly went on Court No. 1 and served a blazing ace down the center line on the first point yesterday. Austin, who ended Navratilova's seven-tournament, 37-match winning streak at the Virginia Slims of Dallas tournament in March, didn't even move for it.

Three more service winners gave Navratilova the first game. Two errors and two double-faults cost Austin the second game. Navratilova won the first 10 points of the match, 14 ot the first 15, and was ahead, 4-0, before Austin - 5-foot-0 and 103 pounds, compared to Navratilova's 5-8 and 145 - seemed to know what was happening to her.

The result from Dallas was far from Navratilova's mind.

"Out her, you don't care what happened before. It's a new ball game," she said.

"I decided to go for my shots right from the beginning. I was too cautious in the first set against Barbara Jordan in my last match, and that's not the way to beat Tracy. I had to overpower her. Just getting the ball over the net wasn't going to cut it because Tracey hits out everything."

Austin got into the match after four games, scrambling for energetic retrieves, buzzing some eye-popping passing shots (especially her two fisted backhands down the line), coming to the net for winning volleys the few times Navratilova did not keep the ball deep, and hoisting some clever lobs.

But she could not prevent Navratilova from swarming the net behind first serves, punshing returns of every second serve, and slicing approach shots off both wings that went deep and stayed low.

Navrtilova was thumping her heavy artillery - serves volleys overheads, topspin, ground strokes - and simply had too much muscle for Austin on this surface. She never lost her hunger for the forecourt or her determined sense of purpose.

"Before Wimbledon, I thought Chris Evert would win the tournament," said Austin. "But now I think Martina has a tremendous chance if her head stays together."

"If her head stays together." Navratilova has heard that phrase countless times in this, the best year of her young career. In addition to the seven tournaments on the Virginia Slims winter tour, she won the circuit championship at Oakland over Evonne Goolagong, her most important title.

"I know that if my head is there , my game is going to be there too. I know that I have the strokes, the ability, the agility," she said.

"I've been good the whole year, I've behaved myself, but people forget that. They remember a couple of years ago. But I'm confident I can keep my head straight.

"It took me almost a year of trying to stay calm. It didn't happen suddenly. I've been trying to believe myself, and in my game. I have a backhand now. It's not a weakness anymore. Tracey tried to hit to it on almost every point, but I stayed on top of her."

Martina was asked if her defection from Czechoslovakia in 1975 was a major factor in her maturity.

"I thind it was just a matter of growing up and gaining experience. I thind it would have happened anyway, but mayne it helped it happen sooner," she said.

Navratilova petitioned the U.S. Congress earlier this year for a waiver of the five-year residency requirement for U.S. citizenship. It was rejected, which saddened her because she is unlikely to see her parents or 15-year-old sister until she gains her citizenship.

She talks to them by telephone, knowing the calls are undoubtedly monitored (We're not talking about politics. We talk about my matches, my dog, my cat, the family"). She hopes that if she gets to the final, they will travel close to the German border from their home near Prague, and see the match on German television.

"You never know what the government of Czechoslovakia is going to do," she said. "My mother has tried to get a visa to visit me in the United States. My father applied for one to come here to England to see me. Their request are always denied."

Since her defection, Navratilova says, her name has been expunged from the government-controlled media in Czeckoslovakia. She is officially a nonperson, as was Jaroslav Drobny, who won Wimbledon in 1954, six years after defecting from the same country.

"The newspapers give only the results from the othe half of the draw," she said. "My name has been in the papers only since September 1975 - a little statement saying I defected because of money.

"They never write that I am winning tournaments, but the people know because they hear the results over Voice of America or Radio Free Europe. But they cannot read it.

"That's why I want to win Wimbledon," she added. "Because if I do, there is no way that Czech press can not say so."