Bjorn Borg, who has proved himself the overall best tennis player on the world's various surfaces, reasserted his absolute mastery of clay courts today by winning the French Open singles for the fourth time, but not until underdog Victor Pecci had stirred a capacity crowd at drizzly Stade Roland Garros with a gallant comeback.

Like a real-life Rocky Balboa, Pecci hung in through a fearful beating, found inspiration from the adoring crowd and gave the prohibitively favored champion some anxious moments before losing a fight he could take immense pride in finishing.

Battling back from 3-6, 1-6, 2-5 down, Pecci won the third set in a tense tie breaker, 8 points to 6, and had the champ scared until he broke serve for 5-4 in the fourth set and served out a 6-3, 6-1, 6-7, 6-4 decision.

Borg, the reigning world champion who turned 23 last week, was at his spectacular best for 2 1/2 sets.He broke Pecci's heart with quickness, consistency and remarkably accurate passing shots, and looked invulnerable as he sprinted to the verge of a quick knockout.

But Borg was broken for the first time when he served for the match at 5-3 in the third set. Suddenly the overflow crowd of more than 17,000 spectators, the largest ever to watch a tennis match in Europe, got behind Pecci and palpably lifted him up, igniting his competitive flame on a damp and gloomy day.

With the aroused audience vibrating the 51-year-old cement stadium with chants of "Pec-ci, Pec-ci, Pec-ci" after every point the handsome 23-year-old won, Pecci consolidated his formidable attacking game and pulled himself back close enough to breathe down Borg's neck.

"I thought I had the match in my hand at 5-2 in the third. But then he started playing better, taking more chances, and I missed a few passing shots. Suddenly it was 5-all," said Borg, the first man since Henri Cochet (1926-28-30-32) to win the world's premier clay court title four times.

"I got to be a little bit scared. I wasn't hitting through my shots and started hitting short, and then he was coming in on every point. He made this the toughest final I have played here."

The 6-foot-3-inch Pecci, ranked 30th in the world, had upset seeded Corrado Barazzutti, Harold Solomon, 1977 champion Guillermo Vilas and Jimmy Connors to become the first unseeded finalist at Roland Garros since Yugoslav Nikki Pilic in 1973.

In so doing, he captivated the imagination of the French public. His darkly handsome face, large diamond adorning the right ear, was prominently featured on the front page of every newspaper. The hard, flat first serve that had zapped No. 3 seed Vilas and No. 2 Conners was repeatedly analyzed in slow motion on French television. The talk of the town was this doctor's son from tennis-poor Paraguay.

Before the final, an army band played "76 Trombones" and French folk favorites. They should have played the theme from "Rocky," for that would have captured the feeling that the audience, huddled under the rain gear against the damp and chill, had for Pecci.

When the players were introduced, the unexpected challenger received a thunderous standing ovation.

"This is normal, I think," the stoic Borg said later. "I had won here three times before, Victor had already beaten Vilas and Connors, and the crowd wants to see a new winner. I expected this."

But Borg - who had lost only 32 games in romping through the normally grueling tournament in 21 straight sets last year - started out playing like a once and future champion.

Pecci tried to play him soft and short, waiting out his opportunities to attack the net and moving Borg around with drop shots and chips, trying to set up a decisive thrust.

But Borg's combination of speed afoot and uncanny anticipation makes him perhaps the quickest player in the game and on the slow, heavy court he was easily running down every drop and approach shot, hitting outright winners or forcing shots off them.

Pecci held his serve from 30-40, after three deuces, for 2-2 in the first set, but then Borg won 12 of the next 13 points.

Pecci was missing his big first serve frequently - he connected on only 13 of 27 in the first set, 78 of 145 (54 percent) for the match, compared with 67 percent against Connors - and when he missed three of four in the sixth game, Borg broke him at love.

Meanwhile, Borg was serving at only three-quarters pace, but placing his deliveries well and missing hardly any - a smart approach on clay.

Borg put a remarkable 100 of 107 first serves in court during the match: 93 percent. He lost only four points on his serve in the first set, five in the second and six up to 5-2 in the third - a total of 15 points in his first 13 service games.

By that time Pecci, who was playing well enough to be in a close match with almost any other opponent, looked confused, dispirited, uncertain as to what he should do.

There is no way to beat Borg, whose topspin ground strokes probably are the steadiest in the game, from the back court. But when Pecci came to the net - behind his serve, or either chipped or thumped approach shots - he was getting passed left and right.

Particularly devastating were Borg's backhand cross-court passing shots, sharply angled beyond Pecci's reach. They invariably bit into the moist clay about three inches from the sideline.

Borg held his serve from 15-30 to lead the third set, 5-2. To that point, he had made a grand total of four unforced errors.

But perhaps he was winning a trifle too easily, for he seemed to let up just a bit in his normally steely concentration. Pecci held at 15 with a blazing service winner down the center.

When Borg served for the match, Pecci intercepted a backhand down-the-line pass with a crackling backhand volley for 0-15. "Pec-ci, Pec-ci, Pec-ci," came the cry, calling for a revival.

Another backhand passing-shot attempt and a blistering forehand cross-court volley winner: 0-30. A backhand cross-court volley off a little backhand angled scoop: 0-40.

Borg saved one break point, Pecci netting his umpteenth backhand drop shot, then whaled a forehand approach. Pecci rifled a backhand down the line, forcing a lunging forehand volley error.

At last, a break. "Pec-ci, Pec-ci, Pec-ci."

Lifted by the crowd's ardor, Pecci started to bore in on the net. Borg, having let up, could not immediately regain his same level of intensity. The passing shots that had seemed radar-guided earlier now were missing.

Into the best-of-12 point tie breaker they went.

Pecci had a set point at 6-5 and got another at 7-6 when a Borg lob on the dead run floated six inches long. Pecci served and came in behind a backhand, and Borg hit another backhand wide down the line.

Coming back from the 10-minute intermission between the third and fourth sets, Pecci felt he had a chance to win. He held serve after one break point and four deuces in the first game of the fourth set and from 15-30 in the third game. Then he missed six of 10 first serves, lost his serve on a double fault to 2-3 but broke right back, Borg missing another backhand down the line on the break point.

As a mist turned into a persistent drizzle, Pecci held his serve for 4-3 from 15-40, after three break points and three deuces. But Borg was holding his serve more easily again now. He held at love for 4-4.

From 15-15 in the ninth game, Pecci missed six consecutive first serves, and this time Borg did not let him off the hook. Two fierce forehand cross-court passing shots got the Swede to break point, then Pecci netted an overanxious backhand off a rally.

Borg, serving for the match a second time an hour after the first, was more purposeful, holding at 15 with a big forehand down-the-line approach followed by a drop volley winner.

He turned to his coach, Lennart Bergelin, seated in the competitors' section, and raised his arms over his head in triumph, heaving a huge sigh of relief. CAPTION: Picture, Bjorn Borg holds French Open trophy. UPI