For a few anxious moments yesterday, it seemed as if the National Tennis Center, freshly minted home of the U.S. Open championships, might have its first Bloody Sunday.

In most major tournaments there is a day when, suddenly and quite unexpectedly, things start to happen. Seeded players get in trouble, simultaneously on several courts or one after another at center stage.

Some of the favorites win, some lose, and the sound effects of underdog-loving galleries echo from several parts of the grounds. A lazy afternoon takes on a sense of urgency and drama, an identity of its own. It has the chance to become a "Remember the time that . . ." day.

The mood seemed right at about 5 p.m. yesterday. Bjorn Borg was struggling against Bernie Mitton, fighting his own serve and an opponent who was much more comfortable on the rubberized asphalt surface of Louis Armstrong Stadium.

Virginia Wade, who has lived mostly on reputation since winning Wimbledon last year already had been tumbled out on Court 5 by Lele Forood, a 22-year-old aspiring law student who holds dual American-Iranian citizenship, 6-2, 3-6, 6-2.

Eddie Dibbs, winner of three tournaments on clay and the most improved of the male pros this summer, had come from behind to win the second set before blowing his match against Brian Teacher, 6-2, 5-7, 6-3.

Stacy Margolin, a 19-year-old left-hander, had sounded another trumpet blast for the ascending teen brigade in womens tennis by ousting the No. 6 seed, Dianne Fromholtz, 6-2, 7-5.

And now here was Borg in a daze, losing the first four games of his match with Mitton, catching up at 4-4 and then double-faulting to lose the set, falling behind 1-3 in the second set, getting the break back and then coming within one point of a 2-4 deficit.

There were perhaps 14,000 spectators from the matinee crowd of 16,486 in the stadium at the time, and they were inspiring Mitton. Borg was hanging by his fingernails. He was serving fitfully (10 double faults, only 44 percent of his first serves in), missing far more passing shots than usual, moving a step slower than normal.

Suddenly his prospects for his first U.S. open title, his dreams of the French-Wimbledon-U.S.-Australian Grand Slam, his 51-match winning streak, all seemed vulnerable.

The crowd thought so. Even Borg thought so: "I thought if I lost that break point for 2-4 in the second set, I might lose the match . . . Best-of-three-set matches are very risky, especially on a fast surface," he said.

Everybody thought Borg was on shaky ground - except Mitton, who had seen him come back from two sets down against Mark Edmondson in the second round at Wimbledon last year and from two sets to one down and 1-3 in the fourth against Victor Amaya in the first round at Wimbledon this year. Borg won both those tournaments. He has won countless matches from desperate positions.

"I never thought I could win the match," said Mitton, 23, a flasy but woefully inconsistent South African Davis Cup player.

He was right. Borg escaped again, winning, 4-6, 6-4, 6-2.

The 22-year-old Swede attacked when he had to - choosing not to serve and volley, but to approach when he got a short ball. He escaped the break point at 2-3, Mitton's advantage, by forcing a lunging forehand volley error. Mitton netted a backhand approach. Borg served an ace to hold for 3-3. Of his 10 aces in the match, five came after this crisis point.

Mitton, serving with new tennis balls, held at love for 4-3. But the next time he served Borg broke him, cracking a backhand cross-court passing shot to 30-40, then approaching and crunching an overhead.

Borg was not out of trouble yet, though. Serving for the set at 5-4, he double-faulted - his ninth - to 15-40. A beautiful backhand down-the-line pass and a Mitton backhand error got him to deuce, but another backhand down-the-line strayed wide and gave Mitton a third break point.

Borg put his first serve into the net. "I know when I am doing this all the time, it is because I am not throwing the ball high enough, but I was not correcting the mistake," he said.

His second serve was nearly another "double." It caught the back line, and Borg won the point when Mitton lobbed long. Mitton, starting to get ragged now, hit first a back-hand and then a forehand too deep.

Set to Borg, and now he was out of trouble. Despite a couple of leather-lunged shouts of "C'mon, Bernie" from the audience, Mitton lost his serve at love in the first game of the third set. Borg ran his getaway sprint to 11 straight points, holding at love.

He never looked really sharp, but he broke Mitton again for 5-2 and served out the march. They say the true champions are the ones who can win even on an off-day.

"My serve let me down a little bit at 4-4, or maybe I was getting tired. That was the crucial game," Mitton said. "He was starting to get more shots in the court, making me take chances."

Borg, who won his second-round match against Heinz Gunthardt Saturday night, had some trouble judging the speed of the ball in the daylight after playing twice under lights. It also took him some time to adjust to the faster daylight conditions.

But he did not mind having a nerve-wracking match. He welcomes it, as long as he wins.

"If I'm going to win a big tournament, I always seem to have a very tough match that I'm lucky to survive, and it's going to come early - before the quarterfinals," Borg said. "That happens 90 percent of the time whenever I am winning a tournament.

"For sure, surviving this kind of a match gives you a little bit of confidence. Now I think I will play better in the next rounds. Maybe it will be the same thing next time. I don't think so."

Borg's next opponent is Harold Solomon, the No. 12 seed and runner-up in last week's U.S. Pro championship on clay. Solomon never has beaten Borg, but Solomon is playing well. He has lost only 13 games in beating George Hardie, Gene Mayer and Gianni Ocleppo.

Dibbs, the No. 5 seed, hit an easy overhead smash into the bottom of the net to lose his serve in the eighth game of the final set. That was the critical break in his defeat by Teacher, a 6-foot-3 former UCLA All America who also beats Dibbs at the Volvo Classic in Washington in March.

Teacher dominated the first set, serving and volleying mightily on the cement-like court that is his most familiar surface. He double faulted three times to lose his serve after leading 4-2 in the second, however, and when he lost that set his chance seemed to have gone.

He kept boring in on the net, however, volleying with great touch and agility as well as power, and eventually Dibbs made his fatal miscue on a smash from practically on top of the net.

Wade, who has won only one tournament this year, a U.S. Open tuneup at Bergen, N.J., played sloppily and tried to come in behind too many second serves and mediocre approach shots against the steady, determined Forood.

Forood, a Standford senior who includes among her hobbies bird watching and citizen's action work for Common Cause, concentrated on getting a high percentage of her first serves in throughout the final set. Her forehand was reliable, her backhand excellent and her resolve unshakable as she recorded the best victory of her career.