The three previous times they had faced each other across a tennis court, Chris Evert had dominated Tracy Austin to the tune of "Me and My Shadow." Tonight, in a match that startled 10,186 witnesses at Madison Square Garden, the shadow got even.

Austin, the 16-year-old princess of the back court, lost her serve twice to fall behind, 0-3, in the first set. Then she astonishingly ran off 10 straight games and routed Evert, the 24-year-old queen of the back court, 6-3, 6-1, in the $275,000 Avon Championships.

Austin -- now 2-0 in the eight-woman, double-elimination playoff -- thus automatically qualified for Saturday's semifinals.

Evert -- 1-1 after beating Englishwoman Sue Barker, then losing to Austin, her first loss in New York (where she has won four consecutive U.S. Open titles) since 1974 -- must win Friday night to reach the semis.

The will play Dianne Fromhltz, who evened her record at 1-1 by thrashing Greer Stevens, 6-4, 6-4.

Wimbledon champion Martina Navratilova (2-0) qualified for the semis by beating Virginia Wade, the 1977 Wimbleton champion, 6-4, 6-4.

Barker (1-1) kept her semifinal hopes alive by dispatching Australian Wendy Turnbull, 6-4, 6-2. She will play Wade (1-1) for the final semifinal berth Friday night.

Stevens and Turnbull, both 0-2, were eliminated.

The pattern, stroke play and result of the Austin-Evert match were all astounding.

In their three prior meetings -- third round at Wimbledon in 1977, semifinals of the Family Circle Cup last April, and quarterfinals of the U.S. Open in September -- Evert had not lost a set.

But after breaking Austin from 40-0 to grab a 3-0 lead in the first set tonight, Chrissie's mind just did not seem to be in the match.

She has attributed uncharacteristic lapses in concentration this season to the pressures of her pending marriage to British player John Lloyd next month, and they seemed very much with her tonight. Longtime observers could not remember her making so many unforced errors.

Meanwhile, Austin, who rivals Turnbull and Evonne Goolagong as the quickest player in women's tennis, took full advantage of Evert's listlessness.

She was a zephyr, running down every ball on the slow, synthetic carpet laid atop the Garden's basketball floor, squealing as she made remarkable "gets" at full stretch to stay in points that Evert though she had won.

Austin's speed afoot and steadiness -- she was far more sure on her groundstrokes tonight than was Evert -- intimidated the woman who has ranked No. 1 in women's tennis for five years, since Tracy was a tot of 11.

Chrissie rushed her shots, and tried to make them too good. Usually impeccable in balanced readiness to stroke the ball, she hit numerous shots from awkward positions, straining to reach balls just inches off the ground.

"She was hungry," Evert said. "I think she wanted it a little bit more, and she was willing to go out there and work hard....

"I wasn't patient at all. I wasn't willing to stay out there and play the way she was," said Evert, who never in her adult career had lost more than eight games in succession.

Austin, now 5 feet 4 and 115 pounds, has more weight of shot than when she last played Evert six months ago. As it became more and more apparent that Evert lacked her usual crispness, both of mind and of stroke. Austin visibly expanded in confidence and boldness.

"The other times, she was a lot stronger than I was. I was weaker then, and she kind of overpowered me from the baseline. Now I can stay with her in the rallies more," said Austin, the high school sophomore from Rolling Hills, Calif, who turned pro in October and is playing the women's circuit seriously for the first time.

Austin was not as surprised as the audience at the utter collapse of Evert's normally unflappable ground game.

"I just tried to keep on playing steady and not think about that," said Austin, who foiled Evert's prematch plan of attacking the second serve by putting 80 percent of her first serves in court.

"I realized that she was spraying some balls, and I thought that if I could stay in the rallies, I could win them."

Austin not only stayed in the rallies, she began commanding them, floating back some shots, ripping others -- especially backhands that she cranked cross-court at sharp angles. The backhand is Evert's stronger side, but Austin hit to it early and often... and it crumbled.

Evert played a sloppy game at 3-0 in the first set, losing her serve on a double-fault. After that, the deluge.

The rest of the first set was awful stuff, dominated by defensive play and unforced errors, but Austin gradually grew in assurance and started hitting forcing shots, and even an occasional volley. She ran down even Evert's best drop shots, several times attacking off them.

Bouncing up and down confidently as she waited to return serve, Austin simply had much more zip than Evert. She covered the court so well that she left Evert no openings, and Chrissie alternately tried to make her shots too good -- missing them by wide margins -- or babied the ball, giving Austin plenty of time to set up and whack it, seizing control of the points. It was surely one of the poorest matches Evert has played.

"My concentration wasn't very good tonight," admitted an obviously disappointed Evert, who lost only three of 59 matches all last season, but already has been beaten five times in 28 matches this year.

"Hopefully in the next month and a half, before the European circuit, most of the tough things will be behind me, the things that I've really been thinking about, and then I'll be okay," she said, referring to her upcoming nuptials.

Meanwhile Austin, who has beaten Navratilova three times but called this "definitely my best win... the one I really wanted," was delighted.

How would she celebrate?

"I guess," beamed Tracy, who said she was in a daze but seemed to have her wits about her, "I'll go out and get an ice cream."