In the closest finish in the history of major tournament tennis, Tracy Austin won her second U.S. Open title in three years with a marvelously tense third set, tie breaker victory over gallant Martina Navratilova, 1-6, 7-6 (7-4), 7-6 (7-1), today.

In a 2-hour 40-minute match that had the record crowd of 18,892 in Louis Armstrong Stadium in a state of delicious seesaw perplexity, Austin came up with the final stroke of tactical brilliance in a spectacular, erratic and completely unpredictable match.

As Austin and Navratilova paused before beginning the first winner-take-all tie breaker in the final match of any major tournament, Austin, 18 -- with the vanilla ice cream smile and the 10-penny-nail soul -- had a sudden intuition.

Or, call it an instant of telepathy.

In the stands, Austin's brother Jeff was muttering and fuming, saying, "If Tracy hits one more short shot to Martina's backhand, I'm going to crawl under the stands."

Suddenly, that same thought came to his sister.

"Every time I hit it short to her backhand, she would chip it deep and come to the net," said Austin. "I said to myself, 'Why do I keep doing that?' "

The reason, of course, is that Navratilova's forehand is, by far, her more feared ground stroke. Austin had been staying away from that wing for 215 minutes as though it were certain death.

Now, at a juncture when every shred of momentum seemed to be with Navratilova, Austin had the confidence and audacity to try a complete shift in strategy and start bombing at Navratilova's strength.

Just moments before, Navratilova had made a splendid back-to-the-wall stand. For the third time in the third set, she had answered a break of her powerhouse serve with a break back of her own. In fact, she had dodged three match points in the 12th game to force the tie breaker.

So, as the crowd anticipated Navratilova's first Open triumph in her first tournament in this country since becoming an American citizen, Austin was cooking up her mind-bending strategy.

On the second point of the tie breaker, leading, 1-0, on a Navratilova volleying error, Austin lashed a forehand deep to Navratilova's forehand for a clean winner, the first outright winner from the baseline that Austin had managed in more than an hour.

Three times in all Austin went hard to the heart of Navratilova's power and three times she won, twice with untouched winners and once with a forced error.

The emotionally high-strung 24-year-old Navratilova was undone.

"She did something unexpected," said Navratilova, who never before had reached the U.S. final. "Those (forehands) are bread and butter shots for me. She just hit real good shots, and I didn't expect them."

The match ended soon as Navratilova served her 12th double fault of the match. Austin did not have any. Navratilova recorded 30 winners and Austin 29, but Navratilova committed 43 errors to Austin's 17.

For Austin, this was the high point of her career to date.

"When I won here at 16, it didn't mean as much to me as it should have," she said, surreptitiously licking an ice cream cone at her victory press conference. "It was just the next stepping stone after all the other national tournaments I had won in other (younger) classifications. Maybe I didn't quite know what I'd done.

"Now, it means a lot more," said Austin, who was swarmed so badly in the first set that she looked like a pathetically out-of-place little girl in pink.

"I've gone to Wimbledon really wanting to win, and lost. I've come back here (as defending champion in 1980), and lost. And this year I had to come back from (a three-month layoff due to a lower back) injury. It was the hardest period I had ever been through and I had some depressing days.

"So, like I said in my (acceptance) speech, this was the best."

Though she lost two tie breakers, Navratilova and her fiery, gambling play won this crowd. They stood and cheered so long that Navratilova, tears rolling down her face, had to wait for her runner-up's silver ball.

This defeat will stay with Navratilova a long time, not only because she won more game than Austin (18-15), but because she had so many chances throughout the last two sets to make a crushing breakthrough.

For instance, at 4-4 in the second set, Navratilova had double break point that could have allowed her to serve for the match. But Austin escaped on two Navratilova errors that are characteristic of the southpaw's flamboyant and necessarily uneven style.

At 5-5 in the second set, Navratilova had three chances at deuce to produce a match point. And each time, in her anxiety, she hashed a relatively simple shot.

In the second set tie breaker, at 3-3, Navratilova whacked a backhand volley long, then netted a forhand approach to give Austin control.

"I had trouble puttin' 'em away when I was ahead, but I could make 'em when I was behind," said Navratilova. "That's unusual, because I usually play better when I'm ahead."

However, when Navratilova was ahead, Austin was behind. And nobody in tennis is tougher when cornered than Austin, who kept the net-charging Navratilova at bay this afternoon by putting 79 percent of her first serves in play.

At exactly 6 o'clock this evening, tennis reached an historic juncture. For the first time, a major championship would be decided by one tie breaker game, rather than the traditional two games that still decide the final set in the Wimbledon, French and Australian championships. At that moment, it was appropriate that Austin be on hand.