It is hard to think how Beverly Sills could have bid a more affectionate or brilliant farewell to Washington as an opera star than she did on Saturday night in the Kennedy Center when she sang Fiorilla in Rossini's "The Turk in Italy."

Not that there are not greater works, tragic or comic in opera-there are. But the "Turk" gave Sills dozens of opportunities to display her vocal brilliance, her gift for deft comedy, and her ability both to dominate a stage and to take her place on it as one among many outstanding artists.

It is a distinct tribute to the company whose direction she will shortly take over, as well as to her longtime friend, mentor, and inseparable colleague, Julis Rudel, who built the company to its present position, that the entire evening was a stellar showing of individual and collective strengths.

Rossini's fun and games were sung in Andrew Porterhs clever, singable and eminently idiomatic translation, nearly all of which came accross clearly with the strange exception of choral passages. The music is vintage Rossini, demanding solo virtuosity and ensemble precision.

What a difference Rudelhs presence makes in the pit. Under his guidance, not only was the essence of style ideally sustained-enhanced by his wit in the harpsichord accompaniments, with their Mozartean infusions-but the intricacies of the score came off perfectly.

Sills took every kind of open delight in her role, properly making fun of everyone in the cast and sharing the fun with the audience which was more than glad to applaud her from start to finish.

With her voice under complete control, fluent and agile as ever, and at ease in every situation, she matched singing and acting with the kind of art that seems second nature. She was fortunate in being surrounded by a cast in which Alan Titus made the poet one of opera's cleverest inventions, Susanne Marsee a biting foil as the gypsy girl, Zaida, singing and acting in excellent manner, and Jonathan Green doing all that is asked of Zaida's friend, Albazar.

Spiro Malas has everything the Selim Pasha of Rossini could want: handsome, resonant voice, fine agility, and the actor's flair. He was closely seconded by James Billings, whose voice and comic gifts were at their best. Alan Kays sang the tough tenor role of Narciso competently, but his light voice is another of many these days that are being asked to give more than they can safely dispense.

By far the happiest production the New York company has thus far presented in its current run in Washington, the "Turk" is blessed with imaginative costumes and sets by John Coklin, and outstanding direction by Tito Capobianco, kept in good shape by Gigi Denda. On every level, Saturday's performance was a matter of top quality. Sills the singer may now have ended her career in Washington. But it is good to knot that she will be back in the future as mistress of the company in which she looked and sounded so wonderful on Saturday. CAPTION: Picture, Beverly Sills and her husband, Peter B. Greenough, by Gerald Martineau