Hana Mandlikova plays for whimsy, Tracy Austin for will. Because whimsy is so precarious, Mandlikova, the most graceful, unpredictable player in women's tennis, usually succumbs to Austin's relentless determination. The only thing they have in common is the back ailments that made two young ladies the old women of the 1982 tour.

In June, Mandlikova beat Austin for the first time, in the quarterfinals of the French Open, and learned that consistency is vulnerable to inspiration. Mandlikova, 20 and the fifth seed, proved it again today, beating the defending champion in the quarterfinals of the U.S. Open, 4-6, 6-4, 6-4.

John McEnroe, No. 1 seed who is seeking to win his fourth consecutive Open, came perilously close to becoming another former defending champion. It took him five sets and 3 hours 36 minutes to defeat sixth-seeded Gene Mayer, 4-6, 7-6 (7-4), 6-3, 4-6, 6-1.

Chris Evert Lloyd, seeded No. 2, lost her first set and the first game of the second set, when her serve was broken by Bonnie Gadusek, then won the next 12 games and her match, 4-6, 6-1, 6-0. Asked the difference between the first set and the rest, Gadusek said, "She started playing like Chris Evert."

But Evert, still feeling queasy from "cheesecake" poisoning, isn't feeling like Evert. She and Billie Jean King, subsequently defaulted their doubles match to Navratilova and Pam Shriver.

Andrea Jaeger (No. 4), who has had headaches since she walked into her hotel room door this week, had problems concentrating and struggled before beating Gretchen Rush, 3-6, 6-1, 6-0.

Ivan Lendl, the men's No. 3, dismissed unseeded Kim Warwick, 6-4, 6-3, 6-1, and Warwick promptly dismissed him. "I can't see him beating McEnroe or Connors," Warwick said.

"I gave 110 per cent," Austin said. Usually, that is enough; it must have been disconcerting to learn it was not. Asked if Mandlikova won the match or she lost it, Austin replied, "She won it. I was disappointed in the way I played the big points because that's where I'm the toughest. If they are going to win a big point they are going to have to work for it."

It has been less than a miraculous year for Mandlikova, who has not won a tournament. The bad back kept her from competing from Christmas through the middle of March. She played one tournament and took another three weeks off.

The match was resumed with Austin serving for the first set at 5-4, after a rain-induced overnight delay. She saved a break point and held for the set. She broke and held to take a 2-0 lead in the second.

But serving at 3-2, she began to weaken. At 30-30, she hit a backhand long and Mandlikova had break point. Austin's normally trusty forehand caught the net, caught the gist of her frustrating year, and slipped back. "The change was when I finally broke her at 3-3," Mandlikova said.

In the 10th game, Mandlikova broke again for the set, on a long point that amply demonstrated her dexterity. Five times, Mandlikova reached back for overheads that Austin returned with defensive lobs. Finally, one of the lobs was too short and Mandlikova put it and the set away.

Mandlikova, who said that she could play 40 percent better than she did today, gave an intimation of that potential on the last point of the third game of the third set. She fell down at the net after hitting her volley and got up in time to hit a backhand volley winner and lead, 2-1.

With Mandlikova leading, 4-2, she served the final two of her seven aces and took a 30-0 lead. She won the game on a service winner to go up, 5-2. Austin held and now she was on the attack. The ground strokes were fiercer, more precise. When Mandlikova missed six first serves, serving for the match at 5-4, Austin pounced and broke.

Many, when Mandlikova fell behind, thought she was broken for good. But Austin fell behind, 15-30, mis-hitting and netting a backhand. She double faulted to give Mandlikova a match point, a very undisciplined, un-Austin, thing to do.

On match point, Austin tried everything to stave off the inevitable. Mandlikova let a defensive lob drop before her, considering her options. She chose an unreturnable forehand. The open was closed for Austin. She walked off the court, trying to look ahead.

After losing the first set, McEnroe struggled back from 1-3 in the second set tie breaker. He tied the tie breaker at 4 with one of his 14 aces and won the next three points, then won the third set with relative ease but lost the fourth when Mayer broke in the ninth game.

In the fifth set, McEnroe gave up only two points on his serve. Still, the difference between them proved to be the width of the net. With Mayer serving at 1-2, McEnroe hit a backhand passing shot that caught the top of the net, hung there with his championship in the balance and fell over to give him a break point. A forehand volley gave McEnroe the break.

In the semifinals, McEnroe will play Lendl, who has beaten him the last five times they have played. "I don't feel too bad," McEnroe said. "I don't want to go out and play another one."