Like many people, John McEnroe Sr. thought the tennis match was over with his son ahead, two sets to love, and up, four games to two, in the third. So he turned off the television and went to pick up his younger son. Then he turned on the radio and "spent the next 4 1/2 hours sweating," he said.

There was a point in that seventh game, third set, when you knew suddenly you were watching something special in St. Louis' Checkerdome. It was the 13th point. Four times, Mats Wilander had denied John McEnroe Jr. the break that would have allowed him to serve for the set. Now, again it was deuce. Wilander did the unexpected, which is expected of champions. He came in behind his second serve, the weakest part of his game, stood at the net as if he owned it, and put away a forehand volley. Not only was he denying McEnroe his game, he was using it against him.

With that, Wilander seized the advantage. McEnroe lost five of the next six points and the set was tied at 4 each. "The guy got really tough," McEnroe said when it ended 6 hours 32 minutes after it began. The United States' No. 1 player had finally defeated Wilander, 9-7, 6-2, 15-17, 3-6, 8-6, and the Swedish team in the quarterfinals of the Davis Cup matches. "He was playing better and better . . . It was a mental effort just to stay out there."

That third set, like the match Sunday night, went on and on. Finally, it became obscured. There were too many spectacular points to make individual imprints on the brain. All that remained were fuzzy, optic-yellow impressions. The implacable Wilander, returning, getting the impossible and making it plausible. McEnroe, acting out, grunting, groaning, complaining, boiling over and, in between, rallying with Wilander on his serve, changing pace, waiting for an opening (he had no other choice). Sometimes he hit winners; sometimes Wilander returned them. "On a court like this, I should have been able to play my game," McEnroe said. "He kept hitting my best shots 90 percent of the time."

"He can run anything down," said Arthur Ashe, the U.S. Davis Cup captain. "Balls John would put away at a Grand Prix tournament were suddenly coming back at him . . . I think the only other players as quick as him would be (Johan) Kriek, (Bjorn) Borg, maybe (Vitas) Gerulaitis."

He is a cool one, this Wilander, this 17-year-old. He never seemed to sweat. When he returned home to Vaxjo, Sweden, after the French Open (after beating Ivan Lendl, Gerulaitis, Jose-Luis Clerc and Guillermo Vilas), he found 50 reporters waiting for him. "If it's just daily, I talk to them," Wilander said. "If it's weekly, I try not to talk to them. I don't think it's so important."

"He's a very calm person . . . But it's too much for him hearing that he's playing like Bjorn," said Anders Jarryd, who beat Brian Gottfried in the first match Sunday and made McEnroe-Wilander the decisive match in the weekend's best-of-five series. "They are two different people. Mats is Mats. Bjorn is Bjorn."

Mats returns home this week to play in the Swedish Open, but will come back for the U.S. Open in September. There, McEnroe, who said he was not prepared for this, will know what to expect. He will remember how Wilander, two sets up in his singles match Friday night against Eliot Teltscher, refused to give in after Teltscher tied the match. He will remember the 31st game of the third set, when Wilander saved two more break points and demoralized McEnroe so that he lost the 2-hour 38-minute set, double-faulting twice.

Perhaps he will remember the fifth and final set, when Wilander came out bouncing on his toes, impassively, while Americans rallied 'round the flag and their Johnny. Maybe, as Ashe said, that was the difference.

Certainly, it was not anything anyone said. "What do you say to a guy after five hours and 10 minutes?" Ashe said. "In any fifth set, a player has got to do it himself. There's nothing that a coach, a friend, a husband can do."

McEnroe held serve to go up, 7-6. But like everything else, it wasn't easy. Wilander demanded McEnroe's best: two aces, one service winner, two remarkable reflex volleys. Reflexively, McEnroe charged off the court pumped and pumping his fists.

The crowd roared one final crescendo. The pitch could not get higher. Nor could McEnroe.

"When you play a match like this, I just feel . . . uptight," he said. "I really want to win for the team. I don't feel like the U.S.A. is looking at me but in a way that's how it is."

He went ahead, 40-15, and beseeched the gods for an end. A weak return of serve postponed it momentarily. But Wilander netted a forehand off McEnroe's strong return on the next point. McEnroe flung his racket to the ceiling and fell into Ashe's arms.

"Davis Cup fifth matches in the fifth set is what tennis is all about," Ashe said. "The only other thing close to it is the fifth set in the men's finals at Wimbledon. The fifth set separates the men from the boys."

Which is hard to do when the ages of those in question are 23 and 17. "Somehow John came out on top," Ashe said. "Somehow."