For singers, this week's semi-finals and finals in the International Competition for Excellence in the Performance of American Music is like the U.S. Open Championships in golf and tennis. More than 430 singers from 20 countries took part in preliminary rounds last spring. Those were held in Washington, Boston, Evanston, Illinois, Los Angeles, New York and Vienna, Austria.

Ten semi-finalists out of those hundreds will sing in the Kennedy Center this week, and by the time the finals are over next Saturday night, they will have divided, if rather unevenly, about $83,500. To take care of the money matters first, that breakdown will be like this:

First prize, with a total value of $65,000, includes $10,000 cash, $5,000 for a second year, a $15,000 recording contract, and a $35,000 management and concert tour. Second prize, $5,000; third price, $3,000. And to each of the seven who do not end up with one of the first three prizes, an award of $1,500.

One of the most unusual and interesting elements in this competition, which is jointly sponsored by the Kennedy Center and the Rockefeller Foundation, is the absence of any age limit. This year's singers range from 25-year-old baritone Sanford Sylvan to 44-year-old baritone Leslie Guinn, whom some Washingtonians will remember as a soloist at the National Presbyterian Church many years ago. The average age is 34 and a half.

As in last year's competition for pianists, each contestant must present a substantial amount of American music. It is undoubtedly this factor which, in this year's preliminaries, eliminated entrants from other countries where English is not the native language.

Both the semi-finals and finals this week are open to the public without admission charge. At the semi-finals, which will be sung in the Terrace Theater at 11 on Wednesday and Thursday mornings, the 10 singers are: James Atherton (the only tenor), Barbara Hocher and Wilma Shakesnider, sopranos, Mary Ann Hart, Barbara Martin, and Glenda Maurice, mezzos, Guinn, William Parker, Charles Roe, and Sanford, baritones. Note the surprising absence of any bass or contralto, and the not-unusual shortage of tenors.

While generous amounts of American music are required of each singer, each one also is free to offer songs from any other song repertoire to make up a sample program. Another item of special interest is that this is not a competition for operatic singing. In the semi-finals each singer will offer a 45-minute recital, the precise contents of which will be selected by the singer and the committee of judges. The judging will be by Jan de Gaetani, George Shirley, Bethany Beardslee, Leopold Simoneau, Max Rudolf, Alberto Ginastera, and Donald Gramm.

The three finalists will sing on Saturday, Sept. 15, in one-hour segments at 6, 7:30, and 9 p.m., in the Concert Hall. These will be broadcast live over National Public Radio.

A regrettable light was thrown on the competition two weeks ago when Ellen Buckwalter of the Rockefeller Foundation telephoned to composer Ned Rorem to ask him if he would present the three prizes to the final winners and also give a short talk. Rorem is the foremost American composer of songs today and two of his cycles, "War and Scenes" and "Poems of Love and the Rain" are among the songs to be heard in the semi-finals. Rorem was asked to come to the finals because his songs had figured so largely in the contest.

But when he asked for an excessively modest fee of $250 to cover his time and effort, Rorem was told that while his expenses could be covered, any thought of a fee was "unprecedented." It is by no means incidental to this unhappy situation that Rorem had no idea that the prize money to the singers would total over $80,000!

In a frankly and justifably angry letter to the Rockefeller Foundation, Rorem said, in part, "It's an old story to us composers, but it still comes as news to everyone else, including well-meaning instigators of competitions for national music: the notion that composers are somehow independent of, rather than dependent on, the music they produce . . . If my reputation, such as it is, centers around some 200 published songs, proceeds from them during any five-year period amounts to less than what certain singers earn for a single recital, a recital which may contain a group of those very songs."

When you recall that no one of the semi-finalists will leave town without having been given at least $1,500, Rorem's scathing response to the Rockefeller Foundation sounds extremely mild. Without him, some of the singers might very well not win top money.