Victor Pecci, an imposing figure with a diamond in his right ear and sparkle in his racket, today upset Guillermo Vilas, 6-0, 6-2, 7-5, and advanced to a semifinal meeting with Jimmy Connors in the French Open tennis championships.

Pancho Gonzalez, one of Connors' early tutors, used to say that the acid test of a great champion was the ability to muddle through and find a way to win on a bad day.

Connors demonstrated that skill today. He was more feisty than ferocious in beating fellow American Eddie Dibbs for the 11th consecutive time, 6-2, 2-6, 6-4, 6-2, even though he was far below his top form.

Vilas - champion of the French and U.S. opens in 1977, runner-up to Bjorn Borg here last year, winner of the Australian Open in January - failed Gonzalez's test on this occasion, but not for lack of effort.

He ran and fought like a man who wanted desperately not to lose, hanging on by his fingernails in the third set, hoping to buy enough time on a painfully obvious off-day for those nails to grow into claws.

But Pecci - the 6-foot-3-inch "Leaning Tower of Paraguay" who had beaten Vilas for the first time earlier this year - would not let the rugged Argentinian left-hander into the match.

He served oppressively, lured Vilas into a serve-and-volley battle, kept him under constant pressure and finally shoved him over the cliff with three winning service returns in the 11th game of the third set.

Pecci, 23, is the son of a former president of the Paraguayan Tennis Federation, the first player of international stature produced by his country.

Tall, dark, muscular and handsome, with a shaggy mane of black hair and a pirate-like flamboyance characterized by that huge diamond glisterning in his ear, he has long been considered a player of rich promise if he could only harness his powerful game and consolidate it with his mind.

He won the French International junior championship here in 1973, and arrived on the men's circuit in 1975, serving a dozen aces on a slow clay court in losing a memorable match to Connors that year at North Conway, N.H., 6-7, 7-6, 6-4.

Pecci has climbed gradually to No. 30 in the computerized world rankings of the Association of Tennis Professionals, but he never seemed to have quite the mental toughness to complement his abundant physical gifts.

Time and again, he lost matches he might have won by playing, first, too impetuously, then too cautiously when the outcome was on the line. The "book" on him, one veteran player said today, was that if you could weather his storms of flashy winners, he would find a way to self-destruct in a jumble of errors and ill-conceived strategy.

That book needs some updating.

Pecci played well last fall and this winter and spring in South America and Europe. He has remodeled his serve, tossing the ball lower and accelerating his racket head faster. This has given him a delivery as powerful as before, but much more accurate and consistent.

He also has demonstrated a new-found resolve and match sense. In the last week, he has played marvelously aggressive and intelligent tennis on the red clay courts of State Roland Garros, beating seeded Corrado Barazzutti, Harold Solomon and now Vilas, all in straight sets.

Today, on the baked but windblown center court of a stadium packed with 17,000 startled onlookers, he was superb. His diamond gleamed in the sun and his game, to borrow the phrase of poet Walter Pate, "Burned with a hard, Gem-like flame."

It was evident from the outset that Vilas, who had to win two sets Tuesday to complete a five-set victory over Gene Mayer which had been suspended overnight, did not have his usual timing or touch.

He mis-hit a discouraging number of shots, could not find the range on his normally deft lob, and bungled one volley after another as Pecci raced to a 3-0 lead.

Vilas had five break points in the fourth game, which went to deuce seven times, but could not capitalize on any of them. He got to 30-all as Pecci served for a love set, but Pecci walloped a forehand winner and served an ace to hold again.

Although few thought then that Pecci could sustain his serve-and-volley onslaughts or that Vilas would continue to make so many errors, the pattern of the match was already set.

Pecci held for 5-1 in the second set after four deuces and one break point, and served out the set from 0-04, after two deuces and four break points.

Vilas had long since abandoned his usual slow-court style of staying back and trying to win long points in trench warfare from the baseline. He was following his first serve to the net every time, trying to fight attack with counterattack.

"For this match, that was the tactic I chose. . . He was very strong, I could not keep him back, so I thought I had to attack the net as well," Vilas said later.

Vilas broke for 2-0 in the third set, but immediately lost his serve at 15 in a game in which he netted two easy forehand volleys - the sort of mistakes that haunted him despite his determination to slash it out with Pecci in a duel for the net.

Thereafter, Pecci lost only six more points in five service games.

Vilas kept battling, luckily escaping a break point when Pecci netted a wide-open passing shot at deuce in the seventh game, saving a break point in the ninth game by chasing down a lob and, running full-speed with his back to the net, flicking a topspin lob winner that left Pecci flatfooted in the forecourt.

That shot, kissed with genius, seemed the sort to save a faltering champion, but it was not to be.

Pecci held at love for 5-all, then broke Vilas at 15 with three backhand return winners on which he smartly played the pace of Vilas serves, blocking the ball twice down the line and once cross-court, just out of reach of Vilas' desperate lunges.

Vilas, standing in to receive Pecci's first serve instead of 10 feet behind the baseline as he had been before, went for big returns as Pecci served for the match. He made two of them to get to 30-all, but two more booming serves got Pecci safely to the semis of the world's premier clay court test, his best showing to date in a major tournament.

Can the beat Connors: Pecci was asked that after the match, but before he could reply, Vilas interupted.

"If he plays like he did today, he has a very good chance not only to beat Connors, but to win the tournament," he said.

Connors plays Pecci and three-time champion Borg faces Vitas Gerulatis Friday. The women's semifinals take over the center court Thursday, topseeded Chris Evert playing Dianne Fromholtz and Wendy Turnbull opposing Regina Marsikova.

Connors will likely have to play better than he did today against Dibbs, whom he has beaten in 13 of 16 career meetings.

Connors started off well today, patiently slugging with Dibbs until he got a short ball on which to approach and put away a volley, but then his timing went off fractionally for long stretches of the last three sets.

This was probably because, on the first sunny day here in a week, conditions were much faster than Connors had encountered before. This was the first time he played early in the day, and the ball was flying more quickly through the warm afternoon air, and bouncing more rapidly off the court than it had in the damp, heavy conditions in which he played his first four rounds.

Connors - annoyed first with repeated commotion in the overflow crowd, then with his own errors and patches of loose play - forgot the impeccable manners that had heretofore characterized his first appearance in Paris in six years.

He made several rude gestures, and swore loudly several times in English, but in the end he regained his patience and overwhelmed Dibbs with the incomparably physical, attacking back-court game that has quickly endeared him to Parisian fans.