Astonishing Victor Pecci, the "Leaning Tower of Paraguay" who previously had toppled seeded Corrado Barazzutti, Harold Solomon and Guillermo Vilas in straight sets, today ambushed Jimmy Connors, 7-5, 6-4, 5-7, 6-3, to reach the final of the French Open tennis championships.

Pecci, 23, has fanned the low flame of smoldering promise into a glorious bonfire of unexpected achievement this week. Sunday, he will have the opportunity to realize what he had considered an impossible dream when he plays defending champion Bjorn Borg for a $49,000 first prize and the world's premier clay-court title.

Borg sprinted into the final with a 6-2, 6-1, 6-0 rout of Vitas Gerulaitis, who was so humiliated by the onesideness of his 12th consecutive loss to the reigning world champion that he shattered a racket on the dressing room floor and stormed away from Stade Roland Garros without showering.

Borg, the champion here in 1974, 1975 and 1978, who celebrated his 23rd birthday this week, was in top form today, making the increasingly impatient and frustrated Gerulaitis look at best like an impetuous rookie, at worst like a fool.

But it was Pecci - the sturdy, 6-foot-3 son of a tennis-playing doctor from a country with virtually no tradition in the sport - who fired the imagination of the sellout crowd of 17,000 on a chilly, dreary afternoon.

At first, the spectators doubted that he could keep pounding the hard, flat first serves that sparkled like the diamond he wears in his right ear, accounting for eight clean aces and clusters of crucial service winners.

They wondered if he could sustain his clever tactics of attacking when he had a chance but otherwise patiently hitting soft ground strokes, making Connors generate his own pace in rallies.

They expected Connors to stop making unaccustomed errors on his back-hand, usually his strong side, and to come out roaring in the fourth set, instead of having his serve desert him at untimely moments.

But when they sensed that Pecci, only 30th in the computerized world ranking list of the Association of Tennis Professionals, was indeed capable of springing another major upset, their excitement and anticipation grew.

There was an eerie hush in the grand old cement arena during fiercely contested points, and an explosion of applause after them. "Unpeude calme, s'il vous plait," intoned the umpire, trying to settledown the blood-sniffing audience. "A little quiet, please."

At the end, when Connors missed a final backhand cross court on the third match point against him, Pecci dropped to his knees and let out a war whoop of triumph. The onlookers broke into rhythmic clapping and chants of "Pec-ci, Pec-ci, Pec-ci," appreciatively acclaiming the first unseeded finalist here since Nikki Pilic in 1973 and the birth of a new star.

"If two weeks ago, if I was asked I would have said it is impossible. But now it is true," a jubilant Pecci said through an interpreter at a trilingual (Spanish, French, English) postmatch interview.

"There is no professional tennis in Paraguay, very few players, but maybe because of my success it is going to start."

The tense, shifting pattern that was to characterize this splendid 3-hour 7-minute match was established right at the beginning.

Pecci broke Connors' serve at love in the first game of the match, jerking him to the net with a couple of drop shots, bashing three winners in a blazing start that confirmed his newfound confidence.

But Connors, full of his customary combative swagger, broke right back, and there were no further service breaks until the 11th game.

Pecci forced a reaching forehand volley error to get to 30-40, then retrieved two grunting Connors smashes, and ran around a backhand to belt a forehand bullet that forced a backhand volley error for the break to 5-6.

Pecci missed six of eight first serves in the next game, but held with four straight points from 15-40, Connors steering a backhand down-the-line approach shot just wide on the first set point.

Pecci broke Connors in the first and third games of the second set, running his streak to six straight games. But Connors promptly stormed back with four in a row, several times wagging his finger at Pecci after intercepting passing shots, as if to say, "Watch it, big boy, I can play this game, too."

If Pecci was infuriated, he got his revenge in the next game. He got to 30-40 with an off-pace backhand that forced a backhand volley error, then chipped a backhand approach and went to the net for two smashes, the second of which Connors lobbed back long for the break to 4-5.

Pecci strode purposefully to his chair for the changeover. Connors stood 15 feet behind the baseline and put his racket between his legs for the first of several prolonged rude gestures. When Connors netted a hard forehand for the set, with Pecci boring in behind a hard forehand approach, he curled his lower lip into a snarl and gestured to the crowd with his finger.

Connors lost his serve at love with three backhand errors in the first game of the third set, but broke right back. He broke again for 5-3, played a sloppy game to lose his serve at 15, then broke again for the set in the 12th game, Pecci twice diving futilely for volleys, finishing with his shirt caked with red clay.

Although Connors looked pumped up when he came out for the fourth set after a 10 minute intermission, his serve was erratic.

He lost his serve from 40-15 in the first game, broke back immediately, then lost it again at love. When he double-faulted twice to lose his serve again to 2-5, the match appeared over.

Pecci was playing smartly. He won numerous points outright with his first serve, even though he got only 67 percent (87 of 129) of them in court. Once he got into rallies he did not try to outslug Connors or get to the net too quickly, as Gerulaitis had against Borg.

Instead, he was all patience, with an undercurrent of aggression.

He continually flouted back sliced backhands and topspin forehands that he hit backing up off the back foot, letting Connors punch himself out in a tennis version of Muhammad Ali's celebrated "rope-a-dope."

Pecci also occasionally followed his serve to the net. Most of the time, he either wacked an approach shot off Connors' return or, if it was hard and deep, rallied until he got a good ball to the net, he volleyed decisively.

Pecci did choke when he served for the match at 5-2, sailing a forehand long at 30-30, then angling an easy smash four feet wide of the court for the break.

But he played a strong, aggressive game on Connors' serve, and got to his third match point with a lucky shot - a mis-hit backhand volley that died like a drop shot. Connors grimaced, then missed another of his usually reliable backhands for the match.

If Connors' gestures during the match were less than well-mannered, he was uncharacteristically gracious in defeat - full of priase for both the tournament, which he had not played in since 1973, and for Pecci. "If he serves and plays the way he did against Vilas and me, he should give Borg a good match," Connors said.

His defeat here seemed to increase Connors' desire to play Wimbledon, which begins two weeks from Monday. CAPTION: Picture, Victor Pecci glories in four-set surprise triumph over Jimmy Connors in semifinals of French Open, world's top clay-court test. AP