Eighteen-year-old John McEnroe, the brash lefthander from Douglaston, N.Y. stormed into the Wimbledon semifinals today in an extraordinary celebration of youth.

In addition to McEnroe, the men's singles semifinalists who emerged today are defending champion Bjorn Borg, 21; Vitas Gerulaitis, 22; and Jimmy Connors, the "elder statesman" at 24.

They are the youngest quartet to reach the semis here and, Borg aside, certainly one of the most flamboyant.

McEnroe, who defeated Phil Dent in five sets today, is, as far as canbe determined given sketchy pre-1900 records, the youngest man to gain the final four of the world's premier tournament.

Schooled at the same Port Washington, (L.I.) Tennis Academy program that nurtured Gerulaitis, McEnroe already has won eight matches, one three qualifying rounds to earn a spot in the 128-man draw.

No qualifier has ever done so well.

Today McEnroe, who is ranked No. 2 among U.S. juniors and No. 270 in the computer ratings of the association of Tennis Professionals, came back from a 1-2 deficit in sets and a break in the fourth to defeat Dent, the first seeded player (No. 13) he has faced here, 6-4, 8-9, 6-2, 6-2.

McEnroe, an amateur who cannot accept prize money here, next plays Connors, the 1974 champion and No. 1 seed.

"I haven't played him. I've never even met him," said the flip McEnroe, who spent his first week at Wimbledon in the "B" Men's changing room, for second-class citizens. He moved in with the big boys in the more lavish "A" room before today's match.

Connors was far below peak form today but still prevailed over Bertram, the junior champion here in 1969 and 1970.

With the match tied at a set each, Connors was 4-5, 0-30 down in the third, but won four straight good points to haul himself out of trouble. He broke Bertram's serve in the next game.

With his small entourage screaming encouragement from the competitors' stand at courtside - his mother, Gloria Connors, yelled "You're the greatest," and Lornie Kuhle, a teaching profriend from Los Angeles, cautioned, "Be alert, he's a no-brainer" - Connors broke the flashy but inconsistent Bertram with a blistering forehand winner from the backcourt.

Bertram, who plays in dreamlike and nightmarish streaks, plodded to 2-2 in the fourth, but was broken after one deuce by a superb Connors forehand down-the-line pass. Thereafter, Connors romped.

Borg, who has grown more impressive with each match, demolished Nastase, the No. 6 seed, in a rematch of last year's final. Borg now has dominated Nastase in straight sets the last three times they have met in major tournaments, twice here and in the semifinals of last September's U.S. Open.

"Nastase doesn't have as much power as he used to, especially on the volleys," said Borg, who passed the mercurial Romanian repeatedly off blocked volleys that, instead of penetrating, sat up invitingly on the spongy turf of the Center court.

Nastase started arguing with officials and doing his unique impersonation of a rope breaking, strand by strand, on the second point of the match.

He won only seven points in the first set and managed to ruffle even the usually impassive Borg in the third, when he drilled a shot at the young Swede and later was lectured and reported by the umpire for spitting, swearing and stalling.

McEnroe threw several tantrums today but never lost his poise as he dug in against Dent, 27, the well-muscled Australian Davis Cupper who had beaten him in five sets in the second round of the French Open.

McEnroe looked as if he felt the occasion only when he blew four match points as Dent served from 3-5, 0-40 in the final set.

On the first three, McEnroe netted weak returns after getting an advantage point with a clean backhand cross-court return winner, having set up Dent a smash. Dent, a semifinalist in the Italian and French opens, served out the game and gave McEnroe the one-minute changeover to think about serving the biggest game of his young life.

"I would have much rather won the game at 5-3 instead of having to serve at 5-4, because I was getting a little nervous inside," McEnroe admitted. "I really choked on those first four match points."

McEnroe fell behind, 15-30, in the last game, but got to match point again when Dent netted a hard backhand down-the-line pass and knocked a forehand return long.

On the final point, McEnroe served away from Dent's potent forehand, and the Aussie knocked his backhand down-the-line return wide. McEnroe raised his arms, ran to the net, and smacked it with his racket in an exultant gesture of determination rewarded.

It had been a long and winding road, this 3 hour, 9 minute match of bizarrely shifting patterns and opportunities earned and wasted.

When McEnroe lost his serve in the first game of the third set, Dent zapping a forehand reverse cross-court pass at 30-40, McEnroe looked disgustedly toward some friends in the court-side stand and changed ends in a snit.

He looked like he would fold then. Chalk it up to inexperience and temper. But he broke right back, and there were five service breaks in seven games.

Dent finally won the set, holding from 15-40 as he served at 5-4, and he seemed in command when he broke McEnroe for a 2-1 lead in the fourth.

At deuce in that game, Dent skitered along the baseline, moving left as McEnroe tossed up the ball and served to the deuce court. McEnroe served down the centre, but Dent had gotten far enough over to run around and whack a forehand winner.

For the break, Dent lashed another vicious forehand across court that McEnroe could only dive at and deflect. He sprawled on the ground, grandstanding a bit, but then climbed up and won four straight games.

"I think he let up a bit there. If he had really put the pressure on, he might have had me," McEnroe said. Dent, who was nursing a strained thigh muscle, agreed.

But in the deciding fifth set, after letting Dent off the hook at 0-40 in the first game, McEnroe was tough. He held from 15-40 in the second game and got the only break of the set in the seventh.

Dent's 13th double-fault, his second of the game, put him at break point for the second time, and McEnroe creamed one of the top spin backhand cross-court passes he hits with panache.

"I was getting a little tired, but when I got the break in the fifth I thought I could win it. I just wanted to get as many first serves in as I could and keep the pressure on," said McEnroe.

Then he squandered those first four match points and looked as if he would pull his frizzy, reddish hair out of both headband and scalp. But he held firm in the last, fateful service game, and is thus the youngest semifinalist in 100 years of Wimbledon.

Gerulaitis, the Italian Open champ, didn't have quite the dash or exuberance he has displayed in earlier matches, but triumphed over his 20-year-old junior rival Martin.

Gerulaitis was a step quicker and a trifle stronger, undoubtedly attributable in part to the fact that Martin played seven sets of doubles that took 5 hours, 13 minutes Monday, finishing at 9:37 p.m.

"I think I ran out of gas, I got tired. I didn't feel as sharp as I had been," said Martin.

"I think I would have defaulted from the doubles instead of playing two matches like Billy did," commented Gerulaitis, who won the doubles here with Sandy Mayer in 1975. "Nobody ever remembers the doubles anyway. People are more likely to remember that you got to the quarters in singles."