It was 8:10 p.m. Saturday at the Kennedy Center and everything was in place for the Boston Symphony Orchestra's 8:30 concert -- except they didn't have a conductor.

Music Director Seiji Ozawa, who had been sick all week, had developed such a high fever that he was flown back to Boston and to bed.

It was the kind of contingency that the Boston Symphony, one of the very best managed of ensembles, normally would have been protected against. Anticipating situations like this, the orchestra gave concert master Joseph Silverstein the additional title of assistant conductor in 1971. But even that precaution failed Saturday night. Silverstein was playing the role of guest artist in the centerpiece of the program, Mozart's heart-rendingly eloquent Cinfonia Concertante in E flat, K 364, in solo duo with the viola of Violist/violin/conductor Pinchas Zukerman.

The result was a kind of instrumental collaboration unlike anything anyone involved had heard of before. In the Mozart, with both soloists standing in front of the orchestra after the conductor's podium had been wheeled off stage, Silverstein set the basic tempi. But when his solo obligations became complicated, Zukerman would take over as conductor. At one point, Zukerman cued the horns by turning around and sticking up the neck of his viola at them.

The performance turned out to be superb. Any slight slips in precision and balance that one would have excused simply failed to occur. As Mozart-playing goes, it was elegantly understated, but there was nothing bland about it.

Interviewed during intermission, Silverstein and Zukerman were asked how they brought the performance off. Said Zukerman, "You have to remember that even in Mozart's day a conductor did not play so central a role as now, and what we were doing would have seemed less unconventional."

Silverstein added, "You also have to remember that this was not just any orchestra, it was the Boston Symphony, and if any ensemble can rise to an occasion like this, it can."

From Zukerman, "I guess the real lesson is that when you get right down to it, all music-making is really chamber music, and that's what was proved tonight."

The remarkable feat by which the Mozart was brought off did not eclipse the rest of the program, which opened with Weber's rarely performed overture, "The Ruler of the Spirits," and closed with Berlioz's spectacular "Harold in Italy." for viola and orchestra. It was a broad interpretation with Zukerman bringing to it a bigger, richer tone than this listener has ever heard before.

At this point Zukerman is very likely the most versatile of all major musicians. One wonders what comes next. Cello? Timpani? One wouldn't put it past him.