John McEnroe and Vitas Gerulaitis, lifelong New Yorkers who live within 15 minutes of the National Tennis Center, today reached the semifinals of the U.S. Open there -- McEnroe by default after just 12 minutes of play, Gerulaitis by sweat after 2 1/2 hours of flashy shotmaking.

McEnroe -- who had won his third-round match by walkover when Englishman John Lloyd defaulted with a stomach virus and sunstroke -- advanced by the bizarre score of 2-1, 0-30, retired, when Eddie Dibbs defaulted with a back injury after playing only 17 points in the first set.

Gerulaitis outdueled South African Johan Kriek, whom he had routed in the quarterfinals here last year, 5-7, 6-3, 6-4, 6-3. This was a flamboyant match, remarkable for the quality of some of its stroke play in a gusty wind, that delighted the evening crowd of 18,090 spectators, larges in the history of the U.S. Open.

The only thing that marred it was the incessant questioning of line calls by the temperamental Kriek, and his running feud with umpire Jason Smith of New York.

"I don't like to be cheated. That's outright cheating, to take points away that I won," Kriek fumed afterward, objecting to the fact that Smith several times overruled linesmen's calls. Kriek made an obscene gesture to the umpire's chair after the match, drawing boos.

With the third-seeded McEnroe and fourth-seeded Gerulaitis joining No. 2 Jimmy Connors, the defending champion, and No. 5 Roscoe Tanner, who upset top seed Bjorn Borg on Wednesday night, there are four Americans in the men's singles semifinals of America's premier tournament for the first time since 1950.

McEnroe plays Connors in what many will regard the defacto final, now that four-time French Open and Wimbledon champion Borg is out, and Gerulaitis faces Tanner Saturday. The last all-American semis, 29 years ago, paired eventual champion Art Larsen against Dick Savitt and runner-up Herb Flam against Gardnar Mulloy.

Dibbs sustained the muscle ailment in his lower back in the fourth set of his three-hour, 6-3, 5-7, 7-5, 6-1 victory over 1977 champion Guillermo Vilas Monday night, but thought he would be able to play until he got on court this afternoon -- after two rain delays.

An early morning downpour and heavy winds associated with Tropical Storm David delayed the start of play, then sprinkles that quickly made the court slick and unplayable further pushed it back after McEnroe and Dibbs twice warmed up. Once, officials even got the National Anthem played before the drizzle.

When the match finally began under fast-moving gray clouds and intermittent sunshine, in swirling winds that blew into and once blew over the umpire's microphone, the long-suffering matinee spectators got to see only a brief overture.

After each player held serve routinely at 15 in the first three games, Dibbs quit while serving at 30-0 in the fourth game because of the injury he had sustained against Vilas. He said he couldn't run to his right.

"I had two days off, and I thought I could play. I didn't know I couldn't run full force until I started playing against John," Dibbs said. "To play against a guy like McEnroe you have to have two healthy legs and a good back, I guess. I just felt I couldn't really run up to 100 percent against him."

When he moved to his right, a pain shot through his lower back and groin to his right leg, Dibbs said, "So why risk further injury?"

Gerulaitis' victory over Kriek, a 21-year-old South African who lives in Florida and is ranked No. 29 in the world, was much more difficult than his 6-2, 6-1, 6-2 romp at the same stage last year. Kriek has improved markedly as a player, but still does not have the temperament to get the most out of his abundant talent as he showed in letting a 2-0 lead in the third set evaporate.

Gerulaitis and Kriek play almost identical games, relying heavily on extraordinary speed afoot, quick reflexes and agility at the net. Both fly swiftly to the net behind both first and second serves on the asphalt-based hard courts that brings out the athletic best in their games. Both play quickly, hardly able to get the ball quick enough to serve. No dawdling, no messing about, except to question calls.

They played a visually spectacular match, liberally spiced with impossible "gets," improbable angles, unlikely exchanges. It was fun, and exciting -- full of flash and dash.

Kriek steadfastly refuses to go for an ordinary shot if an extraordinary one will do. That is one of his problems. He lets his imagination run away with him and goes for a between-the-legs screamer instead of just getting the ball back. He is reckless, and knows not an iota of restraint. Percentage tennis seems to bore him. He makes too many sloppy errors as if average shots were not worth his attention.

He also lets line calls that he dislikes -- and this seems to include every close call that goes against him -- distract him. When he learns some self-control, he will be just like Gerulaitis: one of the best handful of players in the world, though perhaps too light a hitter and too much a gambler to be the best in the world.

The 78-by-36-foot singles court seemed too small, too confining, for these two speedsters, who scotted and skittered here and there, chasing balls into the front rows of seats, whacking eye-popping shots that hit or missed. They were like two sleek and energetic cats chasing each other's tails, sometimes playfully and sometimes nastily battling each other with quick paws and an occasional hiss.

It was marvelously entertaining tennis, like a speeded-up film, but it lacked the glue of consistency. That is not in Kriek's nature -- not yet, anyway -- and so even when Gerulaitis was behind he never looked as if he would lose.

McEnroe, who is 2-6 lifetime against Connors but 2-2 this year and no longer intimidated by the one player who used to psyche him out, is not concerned that he will have played only one singles match in a week going into Saturday's semis. He has kept sharp playing doubles, and yesterday teamed with Peter Fleming -- they are the Wimbledon champions -- to reach the final by beating Marty Riessen and Sherwood Stewart, 6-4, 6-4.

In the other semifinal, Stan Smith and Bob Lutz -- surprisingly chosen over McEnroe and Fleming to reporesent the U.S. in the Davis Cup American Zone final against Argentina in Memphis, Sept. 17-19 -- defeated Australian veterans Fred Stolle and Roy Emerson, 7-5, 3-6, 7-5.

Emerson and Stolle -- combined age, 83 -- won the U.S. National Doubles in 1965-66, when it was played in Boston. They had not played together in the event since, but brought an 11-match winning streak here and added four more to it.