Tracy Austin, 16, a pigtailed killer in a lacy peach pinafore, made poor Martina Navratilova a 7-5, 7-5 victim today and Sunday will play that old married lady, Chris Evert, for the women's championship of the U.S. Open tennis tournament.

Austin was beating balls in the anonymity that goes with being 8 years old when, in 1971, a shy little girl named Chris Evert, 16, won America's heart by reaching the semifinals of the U.S. Open at Forest Hills.

But today, as Austin invented shots, sneaked in shots and left Navratilova helpless with breathtaking two-handed backhand cross-court passing shots the high school junior from Rolling Hills, Calif., said she had only one problem, really, in defeating the current Wimbledon champion.

Austin couldn't help but think about playing Evert.

"Yeah, sometimes I did," she said after confessing that her mind wandered occasionally even during rallies. "No way I should be doing that but . . . "

She smiled and shrugged as if to say kids will be kids. If truth be known, the 5-foot-4, 110-pound package of competitive iron likely knew she had Navratilova well in hand and so could indulge in sweet fantasy right there in front of the 17,664 customers at the Flushing Meadow National Tennis Center.

"If I had to put money on that match," said Navratilova, who now has lost four times in the Open semifinals without once reaching the championship, "I would put $1,000 on Chris and $999.99 on Tracy. Chris has won before (an understatement: Evert is seeking an unprecedented fifth straight title), so Tracy might be more eager."

Austin's victory today may have had its origin at Wimbledon two months ago. There in the semifinals, Austin took a 5-2 first-set lead on Navratilova only to lose that set, 7-5, and the next, 6-1.

Today Austin also led, 5-2, in the first set before Navratilova tied it at 5-all. Not only that, Navratilova was a single point away from breaking Austin's serve at love to take a 6-5 lead.

"I didn't want a repeat of Wimbledon," Austin said later, "or I'd kill myself."

At triple game point, Austin won five straight points to take the game.

"I really messed up," Navratilova said.

"It was a huge game," Austin said. "I tried to just hang in there.'

The key point in that game making it deuce, came when Navratilova, perhaps the strongest player in the women's game, rifled a forehand cross court to the corner, it seemed a certain winner, and if it were, the game was Navratilova's with her needing only to hold serve to win the set.

Incredibly, off that killing shot Austin struck back with a killer of her own. One suspects that America could use Austin's two-handed backhand cross-court passing shot as a model for a guided missile system, so astonishingly accurate is it. Navratilova may have seen the fuzzy rocket returning, but it passed her by too quickly to be intercepted.

Shaken, Navratilova next botched a forehand return of serve and dropped her racket in abject frustration, a frustration unrelieved by a sorry backhand next that lost the game.

Down, 30-love, in the next game, Austin made it deuce with a forehand volley on the line, gained an advantage with another of those guided-missile impossiblities and won the set with a screaming forehand winner off a mighty Havratilova serve.

Austin is no teen-ager on a momentary high. Navratilova lost in the semi-finals here last year to another teen-ager, Pam Shriver, then 16, but while Shriver was a shining meteor, flashing into view and disappearing, Aus- tin is an unblinking star, rising into the tennis firmament to stay.

Three times in eight tries this year, she has beaten Navratilova. At an age when most kids make $5 for mowing a lawn, Aus- tin has earned $258,323. She has beaten Evert twice in three matches this year.

This is why: She can invent a shot (off a net-cord ball, she half-scooped, half-shoveled a return off her shoelaces and past a mystified Navratilova); she can sneak in one (because she always hits her second serve to Navratilova's backhand, she aced the world's No. 1 player with a second to her forehand side), and, most important, she knows, this little girl in pigtails, that she is very, very good.

"I don't think I've grown any," Austin said, "but I am a lot stronger, a lot quicker. Martina doesn't overpower me as much as she used to."

Pause.

"Chris, either."