The United States took a commanding 2-0 lead over Italy in the 1979 Davis Cup final tonight as U.S. Open champion John McEnroe, playing brilliantly, thrashed Adriano Panatta after Corrado Barazzutti sprained an ankle and had to default to Vitas Gerulaitis.

McEnroe, 20, has not lost a set in nine Davis Cup singles matches. He served and returned serve masterfully to rout Panatta, 6-2, 6-3, 6-4, on a medium-fast synthetic carpet at the San Francisco Civic Auditorium.

"McEnroe is a new phenomenon of the tennis game. I think he will become the best player, and be No. 1 for a very long time," said the Italian ace, who had trouble returning serve on the backhand and was intimidated by McEnroe's speed and agility into repeatedly overhitting the ball.

"When I see somebody play like that, I just say: 'Too good.' I am happy for him, because he is a young player and can still improve," added Panatta. "But I think on this type of surface, indoors, he is already the No. 1 player, better than Bjorn Borg. Borg is better on grass and clay."

Panatta's decisive defeat and Barazzutti's default while trailing Gerulaitis, 2-6, 2-3 -- the first retirement in mid-match ever in a Davis Cup final -- put the underdog Italians on the brink of defeat going into the second day of the three-day, best-of-five-match series to determine international team supremacy in men's tennis.

Only one team in the 79-year-history of the cup has come back from a 0-2 deficit to win; the 1939 Austrailians, who won the last three matches to beat the U.S., 3-2.

The Americans could clinch the cup, which they have won 25 times and recovered last year after a five-year drought, with a victory in doubles on Saturday (WETA-TV-26, 5 p.m.).

Although the team captains do not have to name their doubles pairs officially until an hour before the match, U.S. veterans Stan Smith and Bob Lutz are expected to pay Panatta and Paole Bertolucci.

"I think it would be pretty difficult for us to lose the cup now," said McEnroe, who played up to the form he displayed in beating Gerulaitis for The U.S. Open title in September. "Even if Smith and Lutz lost, it's not very likely that Vitas and I would lose on the same day."

McEnroe is scheduled to play Barazzutti and Gerulaitis is paired against Panatta in Sunday's closing singles, but Barazzutti is considered a doubtful participant.

He twisted his ankle badly while trying to make a lunging backhand volley while serving at 1-2, 15-30 in the second set of the opening match.

Gerulaitis had hit a hard forehand passing shot, and Barazzutti leaped toward the ball and came down awkwardly on his right foot, which had been wrapped because of a chronic ankle ailment. He crumpled on the court, grabbing his ankle and writhing in pain.

After a delay of nearly 10 minutes, during which he was attended by the Italian team physician and trainer as photographers hovered over them, Barazzutti gingerly put his tennis shoe back on over a heavy layer of tape and resumed the match.

But "Little Soldier," as the Italians call him, was moving with difficulty. He looked like a wounded tin soldier, hitting off the wrong foot and trying to favor his tender ankle. After losing five points in a row to fall behind 2-3, he surrendered.

Barazzutti was taken for X-rays, but Italian team doctor Girogi Santilli said even before the extent of the injury was determined: "There is not one chance he will play on Sunday."

Under Davis Cup rules, if Barazzutti is unable to play, his place may be taken by one of the other members of Italy's four-man team: Bertolucci or Antonio Zugarelli. "An independent doctor has to state that an injured player is not capbale of playing. They can substitute one of the other members of the team," explained U.S. captain Tony Trabert.

If McEnroe sustains the way he played tonight, it will hardly matter whom the Italians put against him. "We would need Superman," said one of the 37 Italian journalists here to cover their country's third appearance in the Davis Cup final in four years.

"I think the best part of my game tonight was return of serve. I kept him off-guard the whole time," said McEnroe, the gifted left-hander from Douglaston, N.Y. That was understating the case considerably.

On the very first point that Panatta served, he blasted a hard, flat ball into McEnroe's body. It would have handcuffed most players, but McEnroe rocketed it back and forced an error. Nine points later, Panatta lost his serve on his second double-fault of the game. The deluge was on.

McEnroe broke serve again for the set with a blistering backhand down-the-line passing shot, then lost his serves for the only time in the first game of the second set.

He broke right back on the next game, Panatta again double-faulting on game point, and held from 30-40 in the third game, sealing it with two smashing volleys after Panatta netted a running forehand down-the-line.

Thereafter, McEnroe lost only eight points in his last eight service games. Particularly effective was his most natural serve, a deep slice wide to Panatta's backhand, which was good for almost two points a game.

Panatta would have done well to take a leaf from Arthur Ashe's tactical book, as expounded against McEnroe in last year's Grand Prix Master's final, and moved over, forcing McEnroe to serve to his forehand.

But instead, he kept hitting out on every return, and missing them in no caution in his game. It was McEnroe who changed pace and spin, and made sure he got the returns in play. It seemed as if Panatta was anesthetized by McEnroe's swiftness and skill into thinking he had to make every shot too good, an outright winner.

But while Panatta swung wildly, from the heels, it was McEnroe who hit the winners that cut like a scalpel. This was "Mac the Knife" at his best.

Thus an evening that had begun with a bizarre twist, literally, ended with a virtuous performance. The pace and quality of the shotmaking in the McEnroe-Panatta match made that of the truncated opener look pallid by comparison.

Gerulaitis, who served six doubles faults in seven service games and trailed 0-2 in each set, had started nervously, raggedly. He argued a fault call on the first stroke of the day, and was called for a foot-fault double-fault on the same first point.