When a conductor decides to lead a performance without a score, it really means two things. One is obvious: a deep and intimate familiarity with the music. Less obvious, though, is the implication of trust in the musicians of the ensemble. On Friday, young Spanish conductor Pablo Heras-Casado delivered a spectacular interpretation of Mendelssohn’s Symphony no. 3 with the San Francisco Symphony from memory. Infusing the performance with genuine energy, Heras-Casado allowed the orchestra to perform a work its members certainly know by heart as well. His apparent role was to simply guide their energies into an ideal performance.

The first program of a ten-day Mendelssohn/Adès Festival, the music spanned five centuries and four distinct musical eras. Although not without its flaws, the night was a compelling interplay of restraint and ebullience. And despite the less-than-natural pairing of Mendelssohn and Adès, some sense of cohesion magically prevailed.

On the side of restraint were two excerpts from Jean-Baptiste Lully’s 1686 opera Armide. While delivered with an idiomatic coolness, both contained a surprising but subtle undercurrent of melancholy – appropriate for a story about a sorceress who is condemned to eternally love a man who does not ultimately return the favor. The first excerpt, the overture, suffered from a few distracting moments of poor intonation, but the Passacaille made up for it with great clarity.

Adès’ deceptively simple Three Studies from Couperin unfortunately gave the orchestra a little trouble. The first few phrases were muddled, making it difficult to find any coherence in the music. This is perhaps forgivable, though, since the first movement, “Les Amusemens”, naturally evokes a sense of confusion leading to clarity. And despite the outward appearance of a simple melody floating over simple harmony, it is incredibly difficult to play. Both it and the second movement, ”Les Tours de Passe-passe”, are filled with interlocking rhythms that, when performed perfectly, hide their difficulty. In “Les Amusemens”, the melody is shadowed by itself on the off-beat, and eventually the shadow takes prominence in the foreground. The challenge is similar in “Les Tours de Passe-passe”. It takes time, first, to find the melody, and it takes more time still to realize that nearly every note of it is played by a different instrument. As the tempo increases over the course of the movement, it reaches a breaking point where the music dramatically crashes to halt. There was, maybe, too much eagerness to reach that breaking point during the performance, but Hera-Casado managed to keep the orchestra on its feet. After this, the third movement, “L’Âme-en-peine”, was an opportunity to just soak in a lush string arrangement.

If the Lully and Adès were examples of playfulness peeking out from behind a cool façle;ade, Leila Josefowicz’s ecstatic performance of Stravinsky’s Violin Concerto in D major, which followed, was the reverse. Her exuberance was clear even before she made her appearance. As the stage doors opened, the audience could hear her laughing, which continued almost until she was standing in front of concertmaster Alexander Barantschik, waiting to shake his hand, almost cartoonishly. And then, with little pause, she started playing.

And she never stopped having fun. In the opening movement, it was with lively rhythms she vigorously scratched out on her violin. In the second, she uncovered a lyricism rare in any performance of Stravinsky – a composer more obsessed with perfection of form, texture and detachment than melody. The slow movement that followed even bordered on romanticism, and it was only here that Josefowicz became more somber, if no less entranced. For the climactic final movement, she and Heras-Casado revived the earlier playful character, taking the music to an almost Copland-like jaunt. It is hard to find any flaw in Josefowicz’s performance. Even when, multiple times, she turned her back to audience while playing, it was difficult to mind because she and the audience were enjoying themselves so much. It was just an extension of the mood she crafted.

Still, the high point was Heras-Casado’s Mendelssohn. At times, he stood back with arms nearly still, as if he were simply an audience member. At others, he practically pleaded with the orchestra to hold back. But always, his mouth was moving, sometimes audibly singing a melody or grunting in synch with some rhythmic punctuation. If at times this seemed self-indulgent, moments such Carey Bell’s perfectly clear yet perfectly blended clarinet solo in the Scherzo were reminders that the larger musical was being well served.

It was a virtuosic display, plain and simple. For such a young conductor, Heras-Casado’s interpretation was remarkably mature. Even if the rationale behind a Mendelssohn/Adès pairing remains a mystery, his skill and inventiveness make such a detail seem insignificant.