As expected, defending champion John McEnroe and Jimmy Connors won easily in today's Wimbledon semifinals to set up the first all-American final since 1975 and only the third in 35 years.

Devotees of esoterica know this is the first all-left-handed final since Rod Laver beat Tony Roche in 1968. It is, as long as we're at it, the first all-left-handed American final. It will be McEnroe's third straight final here, but Connors' first final in a Grand Slam event since 1978.

What's more, today's semifinals may have been the first in which the losers were simply happy to be in the company of their superiors.

"This has been the best fortnight of my life," said the erstwhile Australian janitor, Mark Edmondson, after Connors handed him his hat, 6-4, 6-3, 6-1.

"I am very satisfied . . . I learned a lot being out there with him," said Tim Mayotte, the 1981 NCAA champion from Springfield, Mass., and Stanford University who was set spinning in a McEnroe whirlwind, 6-3, 6-1, 6-2.

Edmondson and Mayotte earned $18,750 each in defeat.

Loser of only one set in six matches, McEnroe, the No. 1 seed in this summer of Bjorn Borg's abdication, never gave Mayotte a free breath.

"It was a little easier than I expected," McEnroe said. "I knew if I got on top of him early, it would be tough for him to come back. I was hitting the ball pretty well today and I didn't let him off the hook."

In the first set, McEnroe earned the first service break in a rain-interrupted fourth game in which Mayotte staved off three break points before cracking under McEnroe's relentless attacking game.

As McEnroe moved Mayotte from corner to corner, three-time Wimbledon champion Fred Perry told a radio interviewer the poor fellow was lost out there. "Mayotte ought to send up a flare," said Perry, the 1934-36 winner.

Even Mayotte, 21, admitted he was in over his head against McEnroe, the New Yorker who is only two years older by the calendar but decades advanced in pure skill.

"He's a better player," Mayotte said, adding weakly, " . . . right now." Then he said, "He's more solid. He can do so many more things. It's a matter of one player being a better athlete than the other, having better control of the ball."

In 13 previous tournaments this year, Mayotte never moved past the quarterfinals. He is ranked 40th in the Association of Tennis Professionals.

The trip to the semifinals of Wimbledon was also exhilarating for the 17th-ranked Edmondson, who moonlighted as a janitor and odd-jobs worker in Australia before he won the Australian Open in 1976.

"It is fabulous to be out there on Centre Court," Edmondson said later, only three days after grousing no end about being assigned to the outer courts here. One day he asked that the Union Jack be taken down because its fluttering cast a shadow on the court. Wimbledon refused.

Not quite as fabulous, though, was his work against Connors, who only two weeks ago beat him in a tournament at the Queen's Club with the loss of just three games.

Connors, seeded second here, broke Edmondson's serve in the first game and was off to a laugher. Edmondson once was so far out of position on a volley by Connors that, in mock desperation, he flipped his racket behind his back in the direction of the fuzzy missile passing 25 feet away.

On Edmondson's very first serve, Connors cracked a backhand return down the line that the startled Aussie had no time to reach. On the next point, Connors picked on Edmondson's second serve, sending a backhand that Edmondson couldn't pick off his shoetops.

Shaken perhaps, Edmondson gave away a third point on a double fault and lost the game with a botched forehand on Connors' sizzling return of the second serve.

"At Queen's, I became overzealous and tried to outhit him and that didn't work," Edmondson said. "This time I decided not to do exactly the same thing because he can probably return serve better than most players serve. I cut down my pace at first and tried to keep the ball low. This worked for a time, but unfortunately he found an answer for it."

Later in the match, Edmondson threw away the power-hitting game that comes with his heavy-thighed body and thought to trade base line shots with Connors. This is a certain route to pain, and soon enough Edmondson was hurting.

"I played all right today," Connors said, "although he didn't give me any chances to find my rhythm at first. He would throw in his big bombs, then double-fault, then hit his second serve harder than his first. So I stayed back to find a rhythm, and it kind of surprised me; he stayed back, too."

After trailing, 4-3, in the first set, Connors won six of the next seven games.