Gene Mayer, perhaps the most underrated player of pro tennis, put on the first virtuoso performance of the $175,000 Washington Star International Championships last night, manhandling Victor Pecci, 6-0, 6-3.

Pecci, seeded ninth and last year's runner-up, had been superb in reaching the quarterfinals, losing only 13 games in three matches. But against Mayer, who beat him a week ago in the quarterfinals in Boston, Pecci was like a boy playing an adult.

Pecci won only nine points in the 24-minute first set. He had one break the entire match. It wasn't so much that he missed a lot of shots. Mayer just hit one winner after another.

"I learned some things playing him last week," said Mayer, at 24 the seventh-ranked player in the world. "I learned that I didn't have to go for the big shot early on every point, that if I could get him into a rally and attack his backhand he was only going to win about one point in five."

That was the way it was throughout the mismatch. "He may have learned something in Boston," Pecci said. "But Victor Pecci didn't learn anything."

Mayer, seeded third, now faces sixth-seeded Brian Gottfried, a 7-5, 6-4 winner over Zeljko Franulovic.

The other semifinal will match Corrado Barazzutti of Italy, who has yet to play a seeded player in four matches, and Argentina's Jose-Luis Clerc. Barazzutti had to play only seven games yesterday aternoon, winning over Pascal Portes, 5-2, retired. Portes, who ousted Jimmy Connors Thursday, had to quit because of a painful sore blister on his right foot.

Clerc, who has hit his ground strokes with more depth and pace than any player in the tournament, was impressive again in romping to 6-2, 6-2 win over qualifier Richardo Yeaza.Yeaza, who won three matches, just to get into the main draw, played well but was victimized by Clerc's ability to hit the ball at about six different speeds.

In a sense, the same could be said for Pecci. The 6-foot-4 Paraguayan had no answers for the variety of shots Mayer threw at him. He won only nine points in the seven games Mayer served, one of them on a double-fault.

I didn't play that badly," said Pecci. "He just played very well."

Mayer had played very well all year, in fact dating back to October. He has won four tournaments this year and, except for two tournaments when he was hurt, has not lost before the quarterfinals except at Wimbledon. There he was defeated by Bjorn Borg in the fourth round.

"I've just been playing with a lot of confidence," said Mayer, who plays with an oversized racket in order to give his topspin and drop shots more touch. "Most times on the court, I expect to win."

Mayer's semifinal opponent today, Gottfried, barely survived yesterday, although he won in straight sets. "The last game of the match I went all out, played aggressively on every point," he said. "If I hadn't won then I might not have made it, I was so exhausted."

The courtside temperature was 92 degrees and Gottfried, who played in 102 degrees Monday, was dehydrated. "I guess I'm not used to the cold," he joked.

Franulovic, who upset Harold Solomon in the second round, was aware of Gottfried's exhaustion. "I felt like if I could just get one break I would go all the way," he said. "I could tell he was very tired. I felt strong. I could have played five sets."

With Franulovic serving at 4-5 in the second set Gottfried attacked and the tactic worked.

Gottfried got to 30-all with a zinging, unreachable backhand volley. Then Franulovic missed a forehand passing shot to let Gottfried gain match point. "I wanted to get it over right there," Gottfried said.

He followed his return of Franulovic's second serve to the net. Franulovic threw up a semi-deep lob which Gottfried slammed into the corner. Franulovic's forehand return was wide.

Clerc, 21, is one of the world's toughest clay court players. He has a good serve and net game and superb ground strokes. Trying to play aggressively against his assortment of topspin shots, is frustrating. Ycaza tried to rally with him from the back court and, even though he hit a member of outright winners, couldn't do so consistently.

It is difficult to judge the quality of Barazzutti's game. He gained the quarters with routine wins over Geoff Masters, Rick Fagel and Andres Comez. Yesterday, Portes was clearly hobbled from the beginning, sliding around the court on the side of his foot, often not running balls down.

"I just couldn't play," said Portes. "It began bothering me yesterday in the Connors match and got worse last night in the doubles, I tried."

Even though Mayer had to play singles and doubles last night and Gottfried had to play doubles while Barazzutti and Clerc were finished playing in the afternoon, Gottfried and Mayer will play the 2 p.m. match today. The reason: TV, PBS wanted the two Americans for its afternoon telecast. The other semifinal will be at 7:30 . . . Manuel Orantes, the 10th seed here who was beaten by Christophe Freyss in the first round, will undergo elbow surgery next week and will not play the rest of the summer . . . Mel Purcell, 21-year-old conqueror of Eddie Dibbs in the second round, probably will sign with Donald Dell when he turns pro next week.