Guillermo Vilas and Vitas Gerulaitis, the top two seeds, advanced to the singles final of the $200,000 Italian Open tennis championships today with victories that ended amid catcalls and derisive whistles for losers who gave up.

Vilas, the 26-year-old Argentinian left-hander who was runner-up here in 1976 but has never won this second most important clay court title of Europe, had little more than a glorified workout against incapacitated Gene Mayer. Trailing, 3-6, 0-3, and moving as gingerly as a toy soldier with diaper rash, Mayer defaulted because of the painful aftereffects of cramps in both legs.

Gerulaitis, 24, the champion here in 1977, lost the first seven games in a pitiful daze, then regrouped and ousted the third American semifinalist, Eddie Dibbs, 0-6, 6-1, 7-5, 6-3.

Dibbs, who earlier needed only one game to clinch a quarterfinal match against Harold Solomon that had been suspended by darkness Friday evening, grew increasingly frustrated and despondent after letting his big early lead over Gerulaitis disappear.

What little remained of Dibbs' diminishing cool evaporated in the scorching sun as he blew a 3-1 lead in the fourth set. Thereafter, he stopped trying and couldn't seem to lose the last three games fast enough, as a crowd of 10,000 excitable Italians at II Foro Italico watched in disapproving silence or hooted his lack of hear.

It was a pathetic end to a disappointing afternoon in the white marble passion pit that has seen so much gripping tennis. One Englishman in the press box summed it up with cruel succinctness: "It seems as if Dibbs has lost interest in this match," he said, "which any sensible person would have done long ago."

Dibbs' surrender can not be condoned, but it can be partially explained by the emotional energy he devoted to completing the quarterfinal triumph over his confrere, Solomon.

They began the day's activities not on the celebrated Campo Centrale (center court), but out on court No. 4-beneath the parasol pines that rim the amphitheater surrounding the six outside courts here.

This is where they had left off at 7:40 Friday evening, darkness closing in before Dibbs had a chance to serve out a match he was leading by 2-6, 6-3, 6-5.

Solomon had one match point on Dibbs' serve at 5-4, but couldn't seize it. At 5-5, in failing light, they begrudgingly agreed to play two more games. Solomon then lost his serve in a marathon game that consumed nearly 15 minutes, and insisted that he could no longer see. After heated discussions with officials, play was suspended.

They resumed at noon today, the midday sun beating down on the slow, dusty red-clay court, and Dibbs prevailed, exploding a backhand winner on the first match point.

Mayer, who has had a good-enough year so far to emerge from the shadow of his older brother, Sandy, was not expected to play at all today after suffering severe cramps following his quarterfinal victory over fellow American Terry Moor Friday.

He defaulted the quarterfinal doubles match he and his brother were schedule to play, took salt tablets, muscle relaxants and massage and tried to play today. But he summoned trainer Bill Norris after the first game, and it was evident he was hurting.

Mayer - a highly unorthodox player who hits both forehand and backhand with a two-fisted grip, utilizating deception to make up for the unavoidable limitations in reach-tried to conserve energy and move Vilas around with drop shots and dinks.

But the 5-foot-11, 165-pound Argentinian is one of the fittest, strongest players in tennis, and he ran eveything down, forcing Mayer in turn to make painful sprints. Soon enough, the cramp moved to his brain, and he wearied of the fight.

"The muscles on the insides of both legs were getting so tight, I was afraid that I would tear them and be out for a week or two," explained Mayer after ending the match prematurely.

Meanwhile, Gerulaitis, who had planned to eat lunch 90 minutes before his match, had his schedule thrown off by Mayer's default. He wound up bolting his food in 10 minutes and going on court feeling totally disorganized.

He played that way in the first set, spraying balls here, there and everywhere. He had absolutely no feel or control, and was confused as to whether he should try to attack the steady Dibbs from the baseline or press him from the net.

Gerulaitis could not hold his serve the first four times he tried, and by the time he was down, 0-6, 0-1, his blond hair was wet and netted, his fair complexion the shade of a strawberry.

A couple of times, he went to the wrong court to receive serve.

But at 40-30, one point away from sealing his eighth straight game, Dibbs double-faulted. After two more points, he double-faulted again to give Gerulaitis his second break point, then netted a hesitant backhand off a Gerulaitis shot that skipped off the baseline.

Winning that first game rejuvenated Gerulaitis as surely as cool water does a man who has been hallucinating in the desert. He swiftly came to life, scooting to the net at every opportunity, and ruling it with quickness, agility and anticipation.

The crispness and accuracy of Dibbs' ground strokes disappeared, and he became as erratic as Gerulaitis had been in the first set. He lost six straight games and looked as if he were suffering from sunstroke.

Serving for the set, Gerulaitis played an awful game, and Dibs creamed a forehand return of a short second serve to break back to 5-5. But then Dibbs played just as bad a game to lost his serve again to 5-6. He hurled his racket down twice after errors, and bashed it against the net post in exasperation at the end of the game. This time, Gerulaitis held serve for the set.

Both players showered and changed shirts during the 10-minute intermission between the third and fourth sets. But as tournament physician Girogi Santilli said after visiting the dressing room, "Dibbs as no energy left, nothing more to spend."

Dibbs broke for a 3-1 lead in the fourth set, then became horrendously ragged again and ultimately just threw in the towel.