Trey Waltke, 22, a pleasant tennis pro from Los Angeles, walked through the long, brick-colored tunnel that leads from the dressing rooms to the Camto Centrale (center court) at II Foro Italico one afternoon this week.

As he stepped from the subterranean entrance out into the muggy daylight, he was started by the deafening wave of cheers, applause and whistles that cascaded from the white marble slabs that comprise the most striking stadium in international tennis.

The noise was not directed at Waltke. The unbridled adulation was for his opponent, Adriano Panatta, 26, the handsome and dashing No. 1 player in Italy. Panatta is II Campione - reigning champion - of the Italian and French opens, the two most important clay-court tournaments in Europe.

Waltke played a good first set and made a respectable showing before losing, 6-4, 6-3, to the hometown idol, revered as the noblest tennis-playing Roman of them all.

"I wasn't that nervous. I just wanted to make sure I didn't lose my head," Waltke said in the players' dinning room overlooking the outside courts, where a breeze swirled the aromas of flowers, pasta, fruit, pastries and other sweet perfumes.

Watlke slugged from a liter bottle of mineral water, watching some beautiful women drift by, waiting for his doubles match, and recounted how lonely a foreign player feels when he opposes an Italian on that majestic Camps Centrale.

"When you walk out of the tunnel," Waltke said, "it's a unique experience the roar that explodes at you. It's tough to play well against him . . . especially here."

When Waltke beat Dick Creatly on the same court Wednesday, the stands were not packed and the spectators were largely indifferent.

"They clapped for good shots but didn't really pick a favorite," Waltke said. "When they do - if an Italian's playing or they get behind an undergod who's giving somebody a tight - they can just lift a player up."

Waltke experienced that last year, playing an obscure Italian named Vicenzo Franchitti, who annually seems to give an inspired losing performance at Foro Italico. Waltke had to match points at 6-2, 5-3, but with the crowd chanting passionately, "Vicenzo, Vicenzo, Franchitti scratced back to lead, 6-5, in the second set as the match went past the pre-arranged curfew.

"I went to the referee and reminded him, 'you said we'd stop at 7:30 no matter what,'" Waltke recalled. "he said, 'Play one more game,' and I barely held my serve. It was getting pitch dark.

"The next morning I beat him in a tie breaker, seven points to two. It was so early there was hardly anyone there. Franchitti just wasn't the same player without the crowd screaming for hom."

Panatta was seeded No. 1 alhtough there are higher-ranked players here, and was in the opposite half of the draw from No. 3 seed Ilie Nastase, the champion of 1970 and '73. This could be coincidence, of course, but the Italian organizers are famous for making the draw in a backroom, presumably as many times as necessary until the names come out right.

If that was the case, their best-laid plans were shattered when panatta and Nastase both fell in today's quarterfinals.

The enthusiastic and volatile Italians will overflow the 7,000-seat stadium for the finals Sunday, plugging every vantage point and shimmying up flagpoles and three trunks, straining to peer over the top of the stands as do the 17 grimy, grotesque, arresting statues that ring the Campo Centrale.

That would be a marvelous scene for an American television audience that only recently has come to appreciate the special charms of tennis in Rome. NBC-TV is televising the last two days (WRC-TV4 in Washington, 5 to 6:30 p.m. Saturday, to Sunday).

This is not a vintage year by any means. The field - minus such formidable clay-court players as former champions Bjorn Borg and Manuel Oranges, and Americans Harold Solomon and Eddie Dibbs - is much weaker than usual. Instead of customarily gorgeous spring weather, this week has brought gray skies, oppressive stickiness and intermittent rain.

Even so, the Italian Open always has a delicious flavor all its own, derived from the fervor of the crowd, the loveliness of the grounds, the quality of the tennis and the thoroughly Roman disorganization that occasionally makes the tournament seem like a choreographed nervous breakdown.

For years, Rome has a reputation for the most outrageous officiating. Not only were the linesmen fiercely nationalistic and protective of Italian players, but most were old and inattentive as well. One official once explained at a bewildered and irate player - who didn't understand Italian that he (the official) had missed an important call because he had turned around to buy an ice cream.

In Rome, it was never uncommon for the draw to be fudged, the referee to get into raucous hand-waving arguments and the tournament to finish a day or two late despite a week of faultless sunshine.

Even the concessionaries are in on the madness. The price of beverages rises proportionally with the temperature. Incensed at being fleeced for a Coca-Cola, an Englishman once snarled at a colorfully capped hawker, "You're a liar, a cheat, a thief and a crook."

"That's right," replied the salesman, shrugging his shoulders, nodding affirmatively and smiling. "You wanna buy a coke?"

But because the Italian are stylish in their imperfections, because the food is delicious and the ice cream heavenly, because the wine is less expensive than water, because tennis played with pressureless Pirelli balls on slow clay in an absorbing spectacle, and because Rome is the Eternal City and eternally fascinating, shortcoming can be endured.

"I love playing here," said Fred McNair IV of Chevy Chase, Md., who notes that there have been enormous improvements in recent years in officiating, ballboying and tournament organization.

"The setting here and at Roland Garros in Paris is the nicest anywhere in the world. It's exciting playing on the center court. I'm scared. I get butterflies like anyone else, but it's inspiring.

"My first time, in 1975, I was a little in awe of the setting, the statues, the Italian crowd. I had heard about how vociferous they are, how partisan. I had heard about the cheating linesmen and clumsy ballboys. But in three years and five singles matches on the center court, I've had nothing but good experiences," said McNair, who lost in the first round here this year.