John McEnroe gave an impressive demonstration of why he is the world's No. 1 tennis player today, and Jose'-Luis Clerc was almost as convincing.

As a result, the United States and Argentina finished the opening day of play in the 1981 Davis Cup final tied, 1-1, with one thing clear: The hopes of the U.S. team lie squarely on the shoulders of McEnroe.

Backed by a partisan crowd of 8,572 in the Riverfront Coliseum, McEnroe defeated Guillermo Vilas, 6-3, 6-2, 6-2. The U.S. lead lasted less than three hours, however, as Clerc dispatched Roscoe Tanner, 7-5, 6-3, 8-6.

Both losers had one thing in common: poor first serves. In Tanner's case, this is particularly disastrous, since most of his game is predicated on his powerful first serve.

"That may be the worst I've served in 10 years," Tanner said. "I must have missed 90 percent (actually 57 percent) of my first serves. The two areas I wanted to be most solid in were my serving and my first volley, and that's where I was most shaky. It threw my whole game out of whack."

Tanner's nervous, tentative play left the impression that McEnroe not only will have to win the doubles with Peter Fleming Saturday, but will have to beat Clerc in the opening singles Sunday for the United States to win the best-of-five match.

If the teams are 2-2 going into a decisive Vilas-Tanner match, the Argentine would have to be favored because of his vast experience (44 matches) in Cup play. Tanner played his 13th Cup singles match today.

Today's matches were little more than warmups for McEnroe and Clerc. McEnroe never lost his serve and rarely lost his temper in his one hour, 34-minute romp. He broke Vilas at love in the eighth game of the first set, broke him twice in the second set and twice in the third.

Vilas had two chances to break service in the second set. He had McEnroe down, 0-40, in the second game of the set and had another break point in the fourth game. The first time, McEnroe survived with an ace, a drop volley and a lob that Vilas missed.

The second time, McEnroe produced another lovely touch volley.

"The last point in the second game was the only one I really had a chance on," said Vilas, sniffing with a heavy cold and sore throat. "He was in control of the points.

"He played well, especially on the big points. I think on these (fast) surfaces, we have little chance to win. John hit almost all his shots well today."

But while Vilas, the moody member of the Argentine team, was making gloomy predictions, the situation was changing rapidly. Moments before Clerc took the court to play Tanner, word spread among the foreign press that there had been a coup in Buenos Aires.

Almost as quickly, Clerc, who is as lighthearted and outgoing as Vilas is withdrawn, changed the direction of the match. While Tanner was nervous and tentative from the start, Clerc was relaxed, hitting out on all his ground strokes, consistently keeping Tanner off balance as he tried to follow his serves to the net.

"I've played two, maybe three matches this week on fast surfaces," Clerc said. "I was getting all his serves into play. I returned so well, and he started missing easy volleys. I was very calm because I have prepared by playing on this surface (Supreme Court) for a month. I am very confident now."

So is McEnroe. Although he was as self-deprecating as usual, calling himself names on a number of occasions, McEnroe was in control.

Although both players got 55 percent of their first serves in, McEnroe's was a weapon, Vilas' merely a point-starter. McEnroe also came up with several high-kicking winners on his second serve, as he consistently served the ball deep and put Vilas on the defensive.

"I really haven't served well for a month, so I've been working on it pretty hard," McEnroe said. "I didn't think I served all that well today, but it was better, anyway."

McEnroe played down his dominance of Vilas. "He's just not confident on this surface, you can see that," he said. "It took me a while to get going . . . I did think I played the big points fairly well."

At 29, Vilas is not the graceful, slashing player he once was. His serve never has been strong. As a clay-court player, he always has relied on his ground strokes. But on this surface and with McEnroe attacking relentlessly, he had little chance to make the match competitive.

By contrast, Clerc, 23, is improving almost daily. Although clay is still his best surface, his serve has improved so much in the last year that even on a fast surface, he is a dangerous player.

In the second set, Tanner was bothered by some line calls, most notably an overrule in the fifth game. Tanner was serving at 40-0 when he hit a forehand that looked just out. It was called good and, thinking he had the game, Tanner walked to his chair. But chair umpire Youssef Makar overruled the call.

Tanner yelled, and team captain Arthur Ashe -- who barely stirred during the McEnroe match -- argued, to no avail. Tanner lost the next four points and the game, botching a volley on break point.

"I wasn't playing well, never got a rhythm, and then when I seemed about to get into the match, a couple of calls hurt me," Tanner said. "But the calls didn't beat me. Jose' beat me and I beat myself."

Indeed. In the last set, with the crowd cheering and waving flags to try to get him going, Tanner broke in the first game. But Clerc broke back in the sixth game, Tanner again losing a 40-0 lead. After that, Clerc held with ease and finally broke Tanner in the 14th game, with a powerful forehand off yet another weak second serve on match point. Tanner's lunging volley was out, and the match was even.

It also meant that Argentine captain Carlos Junquet may choose to rest Clerc and Vilas in the doubles and play second-stringers Ricardo Cano and Eduardo Bengoechea since McEnroe and Fleming, the world's top-rated team, are heavy favorites. That would leave Clerc and Vilas rested for the final day of singles.