The Wimbledon tennis championships began slowly and soggily today, as intermittent showers forced postponement of nearly two-thirds of the opening program.

But the two men seeded to meet a week from Saturday in the men's singles final, Bjorn Borg and John McEnroe, got off to quick starts on the soft, slippery grass courts of the All England Law Tennis and Croquet Club.

Borg, favored to become the first man to capture a fifth successive title since Lawrie Doherty in 1902-06, changed his usual game to suit the slick conditions. He followed his serve to the net repeatedly to avoid low and capricious bounces off the spongy turf in beating Ismail el Shafei, 6-4, 6-4, 6-4.

Borg, needed just under an hour and a half of playing time to dispose of the Egyptian left-hander, who beat him here in 1974 but is, at 32, a part-time player ranked only 203rd in the world. The execution actually took much longer, however, because maddeningly persistent rain squalls interrupted play for 2 1/2 hours after the first set.

McEnroe, seeded No. 2 to Borg and determined to keep his famous temper in check in an effort to shed the "superbrat" image that made him so unpopular in Britain last year, had the same sort of match against Californian Butch Walts. It was easy but elongated by weather, McEnroe winning, 6-3, 6-3, 6-0.

Jimmy Conners and Vitas Gerulatis, the No. 3 and 4 seeds, also cruised through first-round matches in straight sets. Only 22 of the scheduled 64 matches were completed, though. Four were suspended, and 38 never got on court, despite the addition of three more courts this year.

Connors, another reformed bad actor who seems particularly relaxed, whipped Englishman Richard Lewis, 6-0, 6-3, 6-1.

Gerulaitis, runner-up to Borg in the French Open on slow clay in Paris two weeks ago, walloped a younger Swede, 20-year-old Stefan Simonsson, 6-0, 6-4, 6-2.

Simonsson, who looks like a dime-store Borg clone with his long hair, two-fisted backhand and topskin strokes, was the No. 3 junior in the world in 1978, but he could have been one of the players Gerulaitis was reffering to in Paris when he made this provocative comment:

"All the young guys are now playing the same topspin game as Borg, but nobody's anywhere near as good as he at that game. That's where I think a lot of them are making a mistake. Nobody can volley, nobody really serves that well, they just keep whipping these topspin groundstrokes, and so all Europe is going to develop is a bunch of clay court players that are mediocre compared to Borg.

"There are very few young players coming up who have a game like McEnroe, who can adapt themselves to different surfaces. From what I've seen in Europe this year, most of their young players will do okay on clay, but on grass in England or faster surfaces in America, they don't stand a chance unless they develop a more all-around game."

Today Borg, who at 24 is indisputably the best player in the world on all surfaces, demonstrated his flexibility, playing a grass court, serve-volley game that once would have been unthinkable for him. Never before, he acknowleged afterward, has he followed his serve to the net so consistantly.

"That was because the courts are very soft. If you stay back, it's very difficult to play from the backcourt, especially if the other guy is coming in," said Borg, who was not particularly sharp, but was satisfied with his form.

"The bounce is very low, and the balls stay down there, so it's very difficult to make a passing shot. You have to bend way down and hit it perfectly."

Borg, whose 20-odd hours of grass court practice between raindrops the last 10 days constituted only about half of the preparaton time he would have liked, looked trim and supremely fit. El Shafei, by contrast, has become ample around the middle in his semi retirement. "He's looking more thickset than we remember him," is how one British commemtator put it, charitably.

Borg was not really tested at all. He lost his serve only once, at 5-2 in the third set. The nearest thing to excitement in the match came early in the second set, when Borg mis-hit a return of a powerful serve, and the head of his racket shattered, bringing smiles from both players and a burst of laughter from the center court audience.

Borg now has won 76 of 78 matches since last year's Wimbeldon final, the only losses coming to Roscoe Tanner at the U.S. Open and to Guillermo Viallis (currently hospitalized in Paris, where he had his appendix removed last week) at the Nations Cup team competition in Dusseldorf.

"I don't think I have as much pressure this year as last year. I don't feel it as much. Otherwise, I feel exactly the same, " said the remarkable Swede, now sporting a week's worth of stubble on his face. "Last year, I had a chance to win it four times in a row. Now, for sure, I have a chance to win it five times, but still there was more pressure last year because no one had won four times in modern history."

Play on Centre and No.1 courts began only a few minutes later than the traditional "2 p.m. precisely." The Duke and Duchess of Kent entered the Royal Box at 1:53 p.m., and the 20-minute first set was completed before off-and-on showers caused an almost comical series of covering and uncovering the court. No sooner would the grounds crew remove the tarpaulin, it seemed, than the rain would start again, causing them to run out like Keystone Kops and spread the tarp once more.

By midafternoon, there had been no play at all on the 16 outside courts, and the commentator on BBCradio gave this overview of the activity on the picturesque 12-acre club grounds: "It's all umbrellas, plastic macs, damp strawberries and champagne." The courts were declared playable shortly after 5 p.m.