In the closing minutes of Gustav Mahler's "Song of the Earth," after nearly an hour of intensely passionate music, mezzo-soprano Kelley O'Connor summed up all the despair and hope in one note. Following a steady swell in the orchestra, O'Connor released a heartbreaking high note, a forte summation of the composition in a soaring, resonant vocalism.

In his first concert of the 2016-2017 season, principal guest conductor Donald Runnicles led the Atlanta Symphony Orchestra and guest artists O'Connor and tenor Russell Thomas in an utterly enthralling performance of what is sometimes referred to as Mahler's symphony in songs.

In the five-movement "Song of the Earth," Thomas and O'Connor alternated solo duties. O'Connor has a luxurious depth and weight to her voice, a timbre she can fill with sadness and regret or instill with a playfulness — a natural fit for more emotional fare. Thomas, who served as the ASO's artist in residence last season, sings with a triumphant, heroic force, his pliant voice bouncing lightly through the most joyful and carefree of the symphony's songs.

The ASO never sounds better than when Runnicles conducts Mahler. During “Song of the Earth,” the orchestral backbone lies in the horn section, which Thursday night sounded wonderful. Throughout the composition, the rest of the ensemble waded through thick, Romantic writing; with Runnicles’ guidance, the musicians approached each movement with eagerness and aplomb.

Runnicles opened the program with “A Flock Descends Into the Pentagonal Garden,” a 1970s piece of experiential, impressionistic music by Toru Takemitsu. The dissonant piece functions as a bit of sound art, creating a musical atmosphere that ebbs and flows; the music can die down to nothingness or boil up into a cacophony. Slow-moving and not very tricky technique-wise, the piece is nevertheless a master class in ensemble performance. The writing requires a unified sound from the musicians, who are sometimes playing disparate musical ideas. Often there’s no distinct melody to follow, and the orchestra worked to create captivating music that could have sounded adrift and aimless.

Takemitsu’s composition includes a Western approximation of the sound generated by traditional Japanese instruments. At times, certain sections of the ensemble function as a sho, a small, reed instrument that is somewhat of a cross between a harmonica and an organ. The instrument produces a somewhat haunting, otherworldly mix of pitches. Takemitsu translated the sound of the Japanese hichiriki by way of the oboe — this meant Samuel Nemec had to play his instrument using techniques not widely utilized in Western music. These ersatz sounds grounded the piece in Japanese musical culture.

In the end, “A Flock Descends Into the Pentagonal Garden” could have felt like an unwanted accompaniment to the main course of the evening, but the ASO’s careful attention to ensemble playing made the piece a fitting appetizer to a cherished masterwork.

CONCERT REVIEW

Atlanta Symphony Orchestra with Kelley O’Connor and Russell Thomas

8 p.m. Nov. 17. Additional performance at 8 p.m. Nov. 19. $20-$49. Symphony Hall, 1280 Peachtree St. N.E., Atlanta. 404-733-5000, atlantasymphony.org.