John McEnroe was invincible today.

The young American, flashing the form that won him the Wimbledon championship eight days ago, drubbed Czechoslakia's Tomas Smid, 6-3, 6-1, 6-4, to lead the United States to a Davis Cup quarter-final triumph over the defending Cup championship Czechs.

Jimmny Connors defeated Ivan Lendl, 7-5, 6-4, in today's meaningless second match on the asphalt court at Louis Armstrong Stadium. The Americans won three of the four singles and one doubles match in the best-of-five match series and will play in the Oct. 2-4 semifinal against Australia in Portland, Ore.

The Connors match was meaningless to the outcome, but it provoked the American with another chance to pound Lendl -- ranked No. 4 in the world -- in what he described as a "personal battle." Cup Captains Arthur Ashe and Antonin Bolardt agreed before the McEnroe-Smid match to play best two of three in the finale if McEnroe won to give the U.S. an insurmountable 3-1 lead.

McEnroe, who was defeated by Lendl in the weekend's first match on Friday, dominated Smid from the opening serve. He never allowed Smid to breathe after breaking his serve to take a 3-2 lead in the first set.

"McEnroe felt very badly about the way he lost on Friday, very badly," Ashe said. "And he wanted to atone for that."

McEnroe allowed Smid only seven points in the second set (only one on McEnroe's serve) as he took just 20 minutes to take a two-set lead. McEnroe broke Smid in the first game of the third set and each player held serve thereafter, McEnroe closing the match with two service winners and on overhead smash.

"I played my best, but it wasn't enough," said Smid. "He's too good for me. After the first set, he started to serve better and pass me easily."

The statistics told it all: McEnroe hit 26 forehand winners, Smid hit seven. McEnroe hit 22 backhand winners to Smid's eight. McEnroe double-faulted only once.

"I just wanted to end it," McEnroe said. "I've beaten him easy every time (five) we've played. He's kind of discouraged against me."

When asked what the difference was between his play today and his straight-set loss on Friday, McEnroe said, "Lendl played quite a bit better than Smid. And I'm just getting used to the balls, the heat and the court surface (after playing for two weeks at Wimbledon in cool air and on grass)."

At times today, McEnroe looked as if he had no equal on a tennis court. After allowing Smid one point in the first game of the second set, McEnroe won 12 straight points and 14 of the next 15 in taking a 4-0 lead. He won four straight points to make it 5-0, then four straight again to win the set, 6-1, after Lendl won his only game.

During the 12-point run, McEnroe hit nine backhand and forehand winners, six coming on base line blasts. The 22-year-old left-hander from nearby Douglaston, N.Y., was embarrassing Smid with every conceivable shot. "I played better today than any other time this weekend," said Smid, shaking his head.

"It is tough to lose three days like this," continued Smid, who was 8-3 in Davis Cup singles and 9-1 in doubles before losing nine straight sets to the Americans this weekend. "But I realize how it is to play Connors and McEnroe on this surface."

The most entertaining moment of the first match came when Smid, trailing, 4-2, in the final set, hit a serve that McEnroe couldn't return. But the umpire, without checking with the linesman, overruled, said the serve was out and told Smid to take his second serve.

McEnroe walked to the ad court and acknowledged that the first serve was good. But the umpire insisted on making Smid serve again. So McEnroe, who reluctantly returned to deuce court, purposely fanned the second serve and got a standing ovation for the gesture. McEnroe bowed. Smid won the next point and the game, but that was unimportant.

"That serve was a foot in," McEnroe said. "I hate to see me or anybody get the shaft."

McEnroe, and Connors probably will play against Australia, McEnroe said he will "take three weeks off, maybe four. I'm not going to pick up a racket for at least a couple of weeks. I'm glad this thing is over. I really wanted to win my second match badly, after losing the first one. Now I need a rest. I want to be left alone off the court. I want to see if people can forget who I am."

Connors also said he will take off two weeks before playing another tournament. The Connors-Lendl feud began in January at the Masters when Connors (who is supported by many people who watched the match) accused Lendl of purposely losing the second set of a match in order to play Gene Mayer as opposed to Bjorn Borg in the following round. Connors called Lendl "a chicken."

"I don't like the guy to call me a chicken," Lendl said today, "but I'm not going to try and hit him for that."

Lendl, 21, also walked off the court in his country's first round of Davis Cup play earlier this year. The Czechs had already defeated Switzerland, 3-1, and Lendl was playing a meaningless match against Roland Stadler. Tied, 6-6, in the first set, Lendl "retired."

"If he does that in New York City," said Ashe, "I want to get off the court as soon as possible."

Ashe said Bolardt came up to him before the McEnroe match and asked to let Connors and Lendl play best-of-three sets if McEnroe defeated Smid. Then, Ashe said, Bolardt came to him before the finale and said the Czech team had changed its mind.

It was hairy for about 10 minutes," Ashe said. "I know the rules call for best of three of five, but we had a gentlemen's agreement. I had already told Jimmy he could play best of three. There were a couple of ultamatums given. I'm happy Bolardt convinced Lendl to play two of three."

"I was ready to play best of five," said Lendl. "When they change it you're just not ready for it. I had to concentrate more and change my tactics and I wasn't ready."