The ice age of tennis ended today.

And an era of fire began.

It wasn't the warmth of an autumn evening that melted the ice in Bjorn Borg's competitive heart in the final of the centennial U.S. championship.

Rather, it was the blast-furnace heat from John McEnroe's diabolically diversified arsenal that left Borg's game a damp puddle at the bottom of Louis Armstrong Stadium, 4-6, 6-2, 6-4, 6-3.

This was the day McEnroe not only defeated Borg, but owned him, controlled him, demoralized him and, with an indisputable swagger, defied him.

Perhaps Borg -- rich, married and now a 10-year international veteran -- will find some way to return to Sweden and freeze his will to its former hardness.

That is an issue for the future. At this hour, the incendiary McEnroe, 22 and still rising, has completed a power shift at the top of tennis.

McEnroe has beaten Borg before with huge stakes on the table: in the final of the '80 U.S. Open and this past Fourth of July, when he demolished Borg's 41-match Wimbledon winning streak.

However, this final was the first time that Borg truly lost to McEnroe.

On those other occasions, even when McEnroe threw his arms skyward in triumph, there was a sense that the victory was for one day, and one place, only. Borg played nobly until the very end and was defeated only on the scoreboard, not in the depths of his stomach of ice.

This victory -- McEnroe's third straight Open title -- was a different, more encompassing tale.

He knew it, but had the grace not to insist upon it.

asked if he had now mastered the master, McEnroe said honestly, "That's too complicated to answer simply. I don't think I could beat him on clay. But I'm comfortable against him on all other surfaces."

That was the word for McEnroe today: comfortable on the same court with Borg. No one has truly looked that way for five years. By contrast, it was Borg who looked uncomfortable, tactically confused and at sea.

"He's put so much pressure on himself, especially to win the Open. That's not necessarily the right thing to do. Everybody knows his record here (now 0-10) and how important it is for him to win it. I don't think he needs to keep saying it himself . . . Mayve it doesn't help him," said McEnroe, who added, "Of course, I mean, I'm glad he did play that way (that is, poorly)."

In the two greatest showcases of tennis, McEnroe has, within nine weeks, beaten Borg twice in four sets -- both times after spotting Borg the first set.

Today, Borg had few excuses. In fact, he got most of the breaks.

McEnroe had to struggle through five sets the day before. In contrast, Borg escaped Jimmy Connors in three razor-sharp, shockingly easy sets.

That, one might assume, would help Borg; however, McEnroe turned it to his advantage. "I watched how Connors kept blasting every shot, giving Borg pace, letting him get in a groove," said McEnroe. "I swore I'd change speeds."

Next, Borg, so often a miserable starter, got "into" the match early.

After McEnroe rebutted by winning the second set easily, Borg retaliated by going up a break in the third set, only to see that advantage grabbed back from him, too.

Even in the fourth set, Borg managed the beginnings of a typical comeback -- the kind that unravels nervous foes -- when he negated a dramatic McEnroe service break with one of his own.

That juncture, with McEnroe leading the fourth set, 3-2, was the crisis moment when, for the past five years, Borg would probably have held service to drawn even, and then finally pull away.

Instead, Borg, in effect, broke himself; he went down, 4-2, by committing four errors in one game. With his fans imploring him to "dig deep," Borg suddenly found that his margin of error for beating McEnroe on a fast surface had finally become too miniscule to endure. The demand for splendid passing shots, acrobatic service returns, and continually flawless, deep ground strokes was too great.

In the end, with the crowd pleading for him to act like the old Teen Angel of the mid-'70s, or the Iceman of the late '70s, or the King of Sweden of recent history, Borg gave a listless final bow.

Borg barely held his own serve, after surviving two match points, then managed only one meager point as McEnroe served out the match with authority.

At the finish, the clock showed that McEnroe had dispatched Borg in less time, and with less strain, than Tracy Austin had needed to beat Martin Navratilova the day before.

Borg, normally moderately patient in defeat, made a quick no-comment exit after what may be his most hard-to-swallow defeat. Being dethroned has never been fun.

Anyone who wants to sugarcoat Borg's dash for the parking lot can say it was caused by anxiety in the wake of a threatening phone call. However, the best available information after the match was that Borg made his let's-get-outta-here decision even before being told about the crank call in mid-match.

Borg arrived here talking about how he had to win this U.S. Open to fill out the total portrait of his tennis greatness. This evening, it was McEnroe, the new boss of his sport, who was making similar noises of his own.

"I keep hearing how, to be considered one of the real greats, you have to win the French Open (on clay),' said McEnroe. "That's never been one of my real priorities, but, I think that sometime in the future it will be. I can certainly improve a great deal on clay."

That was McEnroe's backhanded way of saying that on every other surface he is now, and intends to continue being, the ruler of his sport.

As far as McEnroe is concerned, it's all part of the same great thaw that he has brought to what was, until recently, a cold, cold sport.