WIMBLEDON, ENGLAND, JULY 5 -- Wimbledon became a public park today, where one racket was as good as another. Zina Garrison, a 26-year-old from the playgrounds of Houston, defeated top-ranked Steffi Graf over three sets of swelling disbelief to gain the final.

Garrison, quite plainly in the tennis match of her nine-year career, composedly sliced and volleyed her way into history, 6-3, 3-6, 6-4. She became only the second black woman to win a Wimbledon semifinal, Althea Gibson claiming titles in 1957-58, but perhaps more importantly surmounted years of personal struggle and inability to grasp one more victory. She had never before made a Grand Slam final.

She ended the match with an ace, which shot down the middle past an astonished, flat-footed Graf.

"It couldn't have been any better," Garrison said.

The size of Garrison's upset could hardly be stated: Graf had not lost a major semifinal in three years. It obscured second-seeded Martina Navratilova's 6-3, 6-4 victory over No. 4 Gabriela Sabatini, the 33-year-old moving a step closer to her coveted ninth title and attempting to become the oldest champion since 1914. Nobody reckoned on Garrison being her opponent in the final, and later Navratilova said with some caution, "If she beat Steffi she can certainly beat me."

The 21-year-old Graf has been the unassailable No. 1 since August 1987, but today she was outwitted in an intelligent, strategic duel. She was not a graceful loser. Curt in her assessment of the match, she said her play was far below her standard and predicted Garrison would have no chance in the final.

"It's a simple loss," Graf said. "Is it a tragedy, is it a disaster?"

It represented only the third time in 43 tournaments Graf failed to reach a final. She was on a streak of 13 consecutive appearances in major title matches, including her 1988 Grand Slam. However, it was her second straight loss in a major, the other to Monica Seles in last month's French Open final.

Graf had reason to call Garrison a long shot. Navratilova holds an overwhelming 27-1 record against Garrison, her good friend and occasional doubles partner. Garrison also wears Navratilova's clothing line.

"I think it's pretty much Martina's tournament now," Graf said. "I don't think Zina has the game to beat her."

But Garrison had the game to beat Graf, who had won five of six previous meetings, losing only as a 15-year-old qualifer here in 1985. While the West German was bothered by a chill wind, a hummingbird and erratic play, Garrison slowed the pace of the match and drew together all the elements of her quick-footed, sure-handed game. "She played a great tactical match," Graf conceded.

"I wasn't afraid of her power," Garrison said.

Garrison announced that this would be no routine advancement for Graf when she broke serve twice consecutively to take a 4-1 lead in the first set. Graf, as is her way, did not let the set go easily. She pressed Garrison through a three-deuce sixth game, finally breaking for 4-2 with a sharp forehand and a bludgeoned backhand pass down the line. But Garrison broke a third startling time when Graf double-faulted for set point. Garrison engaged her in a slice duel from the baseline, just ticking the line with a hard backhand that Graf lifted wide with her forehand, missing the corner.

There was just one service break in the second set, Graf taking it in the fifth game on a rapid volley exchange and then a staggering backhand pass down the line. It seemed that she would make one of her storied recoveries, while Garrison would swirl to yet another defeat after holding a promising lead.

"After the second set I thought, 'I'm right back in the match,' " Graf said. "I thought I'd be okay."

But Garrison not only refused to yield, she seized control of the third set almost from the beginning. She got the crucial break in the third game with a series of winners.

The issue then became whether Garrison could hold serve in the face of Graf's angry onslaught. "The strangest thing is that I was real relaxed," Garrison said. In the fourth game, Garrison killed two break points, reeling off two service winners. It was Graf's last chance, because Garrison never allowed her another inroad.

"I was ready to go for every shot," Graf said. "I knew her strengths and everything. She didn't miss and make mistakes like she has done before. She played to the end."

In the final game, after a long staring moment, Garrison swung the winning ace the down the middle, and leaped into the air.

"I guess it was meant to be," Garrison said.

Navratilova knew the Graf-Garrison result before she took Centre Court against Sabatini. If the scent of upset was still in the damp, overcast air, Navratilova dispersed it quickly. She broke Sabatini's serve in the second and fourth games with spearing returns for 4-0.

Sabatini, appearing in only her second Wimbledon semifinal, surged back with two straight breaks, the once secure set suddenly jeopardized at 4-3. But Navratilova took permanent control in the interminable eighth game, forcing six deuces before winning it. She never glanced over her shoulder again.

Like Graf, Sabatini predicted Navratilova finally would embrace her ninth title. "She's tough," Sabatini said. "She's very tough." But Navratilova remained hesitant, perhaps even nervous in the face of Garrison's totally unforseen upset.

"Obviously my record is overwhelming," Navratilova said. "But she has done it before. . . . She beat the number one player in the world. I'm not certainly not counting my chickens before they hatch. I'm excited to be there."

Navratilova and Garrison comprise the first American final here since 1985. That's when Czechoslovak-born Navratilova, who became a U.S. citizen in 1981, defeated Chris Evert.

Garrison's victory represents a considerable breakthrough for a player who has long been known as a dangerous upsetter, ranked No. 5, but who has lacked a finishing touch. Her only previous place in the record book was as the last player to beat Evert, forcing her into retirement in the quarterfinals of last year's U.S. Open. Two years ago she knocked Navratilova out in the quarterfinals of the Open, her lone victory in that series.

On both occasions she was beaten easily in the semifinals, which contributed to her reputation as unreliable under pressure. A lack of self-confidence perhaps has been a factor, Garrison a product of public programs who lost her father when she was an infant and her mother in 1983 to complications from diabetes. She suffered from bulimia, and emotional upheavals on the court.

"I think if you believe in yourself, that's all that matters," she said. "The truth was in crucial situations I did give up a lot of matches, but I at least I knew I was giving 110 percent."

Graf is the uncertain player right now, beaten in her last three tournaments. Two of the losses came to Seles, who lost to Garrison in the quarterfinals here.

Graf has grappled with personal problems and ill health, suffering a painful sinus condition and unpleasant tabloid stories concerning her family. She did not attribute the loss to outside circumstances, simply putting it down to the gloomy afternoon.

"I was eager, I was ready, I was everything," she said. "It was just not my day at all."

Garrison's response to Graf's prediction that she is not capable of winning the title match was a cheerful, "Thank you. Compliments like that just make me work harder." She paused, then perhaps summed up her tournament with a shrugging, "You get to a final, anything can happen."