It was another day typical of the first week of the Wimbledon tennis championships: huge crowds, weather shifting from brilliant sunshine to breezy overcast, a big name tumbling on an outside court, an even bigger name surviving a struggle on Centre Court, and "shock news" of an injury traveling by rumor almost as surely as if it had been broadcast over a public address system.

This is Wimbledon, the oldest and most prestigious of tennis tournaments, utterly unique in its bustle, its polite congestion, its electricity.

The lofty reputation humbled on a day when more than 37,000 spectators jammed the All England Lawn Tennis and Croquet Club - and untold thousands waited in lines outside in futile attempts to buy tickets - belonged to No. 6 seed Guillermo Vilas, beaten in the second round for the third straight year.

In 1977, Billy Martin did in the Argentinian lefthander. Last year, it was Tom Okker. And this time, Tim Wilkison - a 19-year-old southpaw from Shelby, N.C., whose acrobatic volleys and dives to the grass for every shot he could possibly hope to reach prompted one baseball fan to remark that, "He plays tennis the way Graig Nettles plays third base."

Wilkison blew the first set after leading, 5-3, took full advantage of Vilas' errant serves in the second and third, and chased his way back from 2-5 in the fourth to win an entertaining battle on Court No. 3, 5-7, 6-2, 6-1, 7-6.

Meanwhile, Jimmy Connors, seeded third behind defending champion Bjorn Borg and John McEnroe, labored fitfully on the Centre Court and twice nearly fell behind by two sets to one before dispatching veteran Marty Riessen, 6-7, 6-3, 7-6, 6-0.

Connors' spotty form fueled speculation that he is short of grass-court preparation this year, and that his mind is far away, in Los Angeles, where his wife is expecting their first child any day.

Certainly the champion of 1974 and runner-up to Borg the last two years will have to play better than he did today to win the title. But so, too, will Borg and McEnroe, who were both vulnerable Wednesday.

Moreover, clouds were cast today over Borg's chances to win a fourth consecutive singles title when his coach, Lennart Bergelin, said a strained thigh muscle he suffered in his first-round match against Tom Gorman had worsened, and might force the 23-year old Swede out of the tournament.

Borg had planned to rest today, but he practiced . . . and Bergelin said his injury was "getting worse." The coach said Borg would play his third-round match against Californian Hank Pfister Friday, "but he may not finish . . . the chances are only maybe 25 percent that he will finish the tournament."

Such questions about Borg's fitness in major tournaments are almost an annual occurrence. In 1976, when he won his first Wimbledon title without losing a set, he said before each of his last four matches that he would probably have to default because of a torn stomach muscle.

In 1977, he defaulted with an injured shoulder at the U.S. Open. Last year, in losing the U.S. Open final to Connors, he was bothered by a blistered thumb.

It is difficult to tell when the reigning world champion is legitimately handicapped and when, perhaps subconsciously, he is crying "wolf."

But news of Borg's physcial problems spread quickly at Wimbledon, and undoubtedly will be reflected in the morning lines of the bookmakers who had made him an 11-10 favorite to retain his title.

A player with an injured groin muscle - No. 4 women's seed Tracy Austin - got an erratic performance out of her system today. She moved a little hesitantly in the first set, then "got really made" and beat South African Brigitte Cuypers on the Centre Court, 4-6, 6-1, 6-2.

Austin, 16, who had a first-round bye, had requested an extra day's rest before playing her opening match because of the injury that forced her to default in the quarterfinals of a tuneup tournament at Eastborne, England, last week.

"I didn't move that well in the first set," said Austin, who didn't pick up a racket for three days and ran flat out in practice for the first time Wednesday. "I guess I got used to limping, but it has healed. It feels fine. I moved, played and concentrated a lot better in the last two sets."

Vilas and No. 13 seed Manuel Orantes - beaten today by heavy-serving Gilles Moretton, the tall Frenchman who looks remarkably like heavy-weight boxer Joe Bugner - 7-6, 3-6, 7-6, 3-6, 6-1 - were the latest of the "clay-court seeds" to fall. Jose Higueras (No. 14) and Corrado Barazzutti (No. 16) had preceded them.

Orantes and Higueras usually do not play Wimbledon, and entered this year only because they are preparing for a Davis Cup series against Great Britain on grass two weeks hence.

Vilas, despite his dismal record at Wimbledon since reaching the quarterfinals in 1976, is a different case, however. He can play on grass courts, as he proved by winning the Grand Prix Masters at Melbourne in 1974 and the Australian Open in January. What he needs is more preparation.

"In Australia, I practiced on grass for four weeks. Here I had two weeks, but I need more time," said Vilas, who has to adjust radically the baseline game, founded on heavy topspin ground strokes, that won him the 1977 French and U.S. Open titles on clay.

Wilkison, son of the city manager of Shelby, is an aggressive, tenacious, scrambling player. He hurls himself around the court, energetically chasing every ball, winding up with grass stains all over his "whites" and lunging for winners certain to discourage an opponent.

A natural serve-and-volleyer who gets to the net very quickly, he is at his best on grass - as he proved by winning the New South Wales Open in Sydney last Christmas Eve. Wilkison went through a dismal year after turning pro at 17, but has improved markedly the last two years and is now among the top 50 players in the computer rankings.

Wilkison lost to the Argentinian decisively on a slower indoor court at Richmond last winter.

"I saw him play Roger Taylor on television earlier in the week, and noticed that he gets really close to the net on grass. I don't think he's that comfortable up there, but he's pretty effective because he's so strong. He can knock volleys off from that close. So I tried to lob as often as I could.

"I knew what I wanted to do: chip into his forehand volley - that's his weaker side - and put a lot of pressure on his second serves. I just tried to slice my return and come into the net, to make him hit a shot. If he passed me, okay. But I'd rather make him hit the passing shot than try to compete with him from the back-court."

Wilkison, who had not won a match at Wimbledon before, served for the first set at 5-3, but won only four points in the next four games.

In the second and third sets, he played resourceful, attacking tennis, punishing Vilas' suddenly balky serve, getting to the net for slashing volleys. And when he broke back from 2-5 in the fourth set - with the help of two costly Vilas double-faults - and got into the decisive tie-breaker, he took advantage of what he had learned.

Wilkison did not lose a point on his serve in taking the tie-breaker, 7-3, he got to 3-2 with a beautiful forehand down-the-line pass of a high Vilas backhand volley, reached match point when Vilas netted an easy forehand volley, and sealed it with a backhand cross-court volley.

As that shot clipped the net and landed for a winner, Wilkison dropped his racket and leaped high in the air. The way he had played, one half expected him to dive head-first over the net to shake hands.

Riessen, at 37, the second-oldest player in the men's singles, also played determined, athletic tennis in his bid to upset Connors.

Connors' former manager, Bill Riordan, had placed a bet on Riessen at 5-to-1 odds because he felt Connors looked distracted and preoccupied in his four-set opening victory over light-hitting Frenchman Jean-Francois Caujolle.

Connors started listlessly today, as well, serving poorly, overhitting his forehand frequently and mistiming a number of returns. He lost his serves in the fifth and seventh games of the first set, but each time broke straight back.

Riessen cramped a thigh muscle late in the first set, and although he said it only annoyed him, for the rest of the match he grabbed the umpire's chair and did stretching exercises at changeovers. Nevertheless, he won a tie-breaker, 7-5, to take the set.

Connors tightened his game considerably thereafter, but Riessen also played above himself - especially in climbing back from 1-6, 30-40 to tie the third set and have a break point on Connors at 4-4.

Three sizzling Connors service returns in the third-set tie-breaker, and an easy backhand volley that Riessen missed at 4-5, gave Connors the pivotal game, 7 points to 5. After that, Riessen was exhausted and went out rather meekly, while Connors regained his old killer's strut and swagger.