Wimbledon champion Bjorn Borg, who had won three matches depite a strained muscle underneath his right shoulder, defaulted from the U.S. Open tennis championships today in the third set against Dick Stockton.

Borg had won five games in a row from 1-3 down to win the first set, 6-3. He lost the second, 6-4. After losing his serve in the first game of the third, he looked forlornly to his coach, Lennart Bergelin, seated in the courtside marquee, and pointed to the shoulder.

At the change-over, Borg sat down briefly, picked up a racket, threw it down dejectedly, then got up and told umpire John Sternbach of Denver that he could not continue.

"He came to the chair, got my attention, put his hand on the shoulder and said, "Hurts too much." Sternback said. "He put out his hand to shake, and that was it."

The crowd of about 10,000 at West Side Tennis Club was astonished. So was Stockton, who knew that Borg was in pain, unable to hit out on his serves and overheads, but had no idea that the No. 1 seed was ready to throw in the towel.

"I had no inkling that he was going to retire until he came up to me and said, "I cant play anymore'," Stockton said. "I just said, 'Are you sure you can't continue?" He said, 'Yes, I'm sure.' So I said O.K.

"I would have liked to have seen the match finished, but I guess the only thing that really matters is whose name is written on the scoreboard, and that's mine," added Stockton, who felt he was playing the best taken command.

"It's always unfortunate when someone has to retire in match . . . It always takes a little bit away. Sure, people are always going to say, 'Well, he defaulted, you didn't beat him."

"But I guess I'll take it any way I can get it. This seems to be the year for people to default to me - Jimmy Connors twice (Toronto and WCT Tournament of Champions) and now Borg."

"It was too much pain to serve all the time," said Borg, 21, who insists publicly that the sustained the inlury (strained pectoralis muscle, in his upper right chest) in a practice session last Tuesday, but told other players that he wrenchd it water skiing and aggravated it in practice.

"While you're playing, you think about it all the time. It's a mental thing. Every time before you hit the ball, you think whether it's going to hurt or not," added Borg. "It was getting worse every game.

"I'm very disappointed because I wanted to do really well this year. I was very close to winning last year (runner-up to Connors)," said the Swede, who won the French Open in 1977-75, Wimbledon in 1975-77, nut covets the U.S. Open title as the biggest one he has not won.

"I was playing very well in practice before this happened. I have Plenty more years to come, but I'm still very disappointed."

Stockton, who had settled into a rugged match and was playing smartly and well, had beaten John Alexander and Adriano Panatta, each in three sets, in two of the best matches of the tournament so far. He now meets Harold Solomon of Silver Spring, Md., in the quarterfinals, the stage at which the men start playing best-of-five sets.

Also advancing to the quarters this afternoon were No. 5 seed Manuel Orantes, the 1975 champ, who beat John McEnro, the 18-yeaold Wimbledon semifinalist, 6-2, 6-3, and No. 3 seed Brian Gottfried, who scraped past Wojtek Fibak, 6-7, 6-3, 6-4.

The Stockton-Solomon winners will play either Guillermo Vilas or Ray Moore in the semis. oGtttfried's quarterfinals opponent will be Corrado Barazutti and Orantes is paired against the winner of tonight's match between defending champ Connors and Roscoe Tanner.

Chris Evert, the women's champ of 1975-76 and No. 1 seed, won her 110the consecutive match on clay, eliminating old rival Nancy Richey, 6-3, 6-0.

Evert, who has not lost to clay since August, 1973, while winning 22 straight tournaments on her favorite surface, finally got ahead of Richey, no 35, in their six-year rivalry. Richey won the first five to rank as Evert's foremost nemesis early in her pro career, but Evert has now won the last six.

Evert meets her other rival of hte previous tennis generation, Billie Jean King, in the quarters.

The other women reaching the quarters today were No. 2 seed Martina Navratilova, over Marise Kruger, 6-2, 6-1; No. 3 Virginia Wade, The Wimbledon champ, over Mona Guerrant, 7-5, 6-4; No. 10 Mima Jausovec, the French Open champ, over Dianne Fromholtz, 6-3, 6-2, No. 12 Wendy Turnbull, who upset No. 6 Rosemary Casals, 4-6, 6-0, 6-0.

After losing his serve four times in a row from 3-1 in the first set to 1-1 in the second, Stockton decided to mix up his game. "At first I was just hitting my serves one speed, fairly flat and coming to the net," he explained.

"It worked for two games, but then he moved back and started hitting the ball at my feet.

Stockton, an attacking player, found himself netting or punching long a maddening number of volleys off Borg's dipping returns, so he decided to stay back and rally, only coming to the net when he gor a short ball or hit a good approach. He started spinning in first serves and waiting out chances to come up.

Borg's serve and overhead deteriorated progressively, but his ground strokes did not seem at all inhibited.

"It's still hard to break his serve, even though he was just pushing it in, because he hardly middle of the second set," said Stockton. "No matter how soft it is, you can't just hit the ball and come in because his passing shots are so deadly.

"Except for the serve, I thought he, was hitting the ball well . . .From 1-3 in the first set, he rally started hitting with a lot more pace and depth. The ball was coming at me, just pushing me back, pushing me back, and it relly started to wear me down.

"But then I got a second wind. I felt I was hitting the ball off the game as well I possibly can, coming in on the right shots, putting away volleys and drop volleys, doing everything I could possibly want to do on a clay court."

Perhaps borg sensed Stockton's expanding confidence as well. "It didn't make any difference," he said. "If I win this match, the next one is best-of-five, and there is no way I can keep on going like this."

Stockton tried to play to Borg's forehand, making him stretch, and fed him more lobs than previous opponents (Trey Waltke, John James, Onny Pary).

"I had to force myself to exploit his injury," said Stockton. "It's tough to deo, especially when you're playing one of he nicest guys around, but it's more or less the same as taking advantage of any weakness."

Stockton, who has fought a chronic back problem for several years, missed Wimbledon in 1975 because of it, and knows what it is like to play in pain, was sympathetic.

"It always hurts to default, no matter what tournament it is." he said. "I've never had to default in a major championship like this, and I'm sure that's even more of a dissapointment . . .But your health is the most important thing because you want to be able to play next year, too."