Barbara Cook/John Pizzarelli at Symphony Hall

"I am working with Barbara Cook," John Pizzarelli said from the Symphony Hall stage on Saturday night. "I had better be at my best game." He wasn't joking. If there's a standard by which great singing can be measured, it is Cook - a national treasure at 85.

The self-effacing Ms. Cook may not appreciate giving her age away since she looks twenty years younger and would have every right to lie about her age. Looking radiant in a teal-and-black jacket and slacks, she did something perfectly in character: ventured out on a new musical front. Long famous for singing the best of Broadway, she took the occasion to join Pizzarelli and his trio in a gently swinging set. When in Rome...

Indeed that Cy Coleman-Carolyn Leigh song would have fit nicely in the program. Perhaps next time the Celebrity Series of Boston, who sponsored the concert, brings them back to town. The pair are obviously having a blissful, artistic romance that made for one of the most pleasurable and touching concerts in some time.

When Cook offered a gently rhythmic "Let's Fall In Love," it had me wishing Cook would have done a set with Nelson Riddle back in the 1950s. Instead she was Broadway's leading ingenue with a habit of starring in vehicles often celebrated, but short-lived ("Candide," "She Loves Me"). Upon retiring from the stage, she became unparallelled practitioner of the Great American Songbook.

Adding jazz to her repertoire (or "oeuvre," a word that left her giggling uncontrollably) adds another facet to her remarkable career, one that many of us got to know from hearing her "Glitter and Be Gay" from the original cast album of "Candide," and marveling at her vocal prowess and comic skill.

There were plenty of standards in the set, plus a good many lesser known gems from the 1920s and 1930s, that fit Cook's honey-soaked contralto perfectly. The long melodic lines of songs like "Tea For Two" and "The Very Thought of You" were beautifully expressed by the duo in counterpoint; but it was Cook's swinging side that gave the program its delightful punch, most evidently in "Just You, Just Me" and a splendid "I've Got Rhythm" (which had Cook riffing on a theme from "An American In Paris" during the vocalese).

It was Pizzarelli's 53rd birthday, which gave the evening a celebratory feel. His solos, which included the ever-so-clever "Rhode Island is Famous For You" and "As Time Goes By," were spot-on. In addition to the well-chosen song selection, there was banter about such topics as how Paul McCartney's vegetarianism leads him to change lyrics, Cook's first New York crush and Pizzarelli's trial-by-fire apprentice under her the tutelage of his dad, guitarist Bucky Pizzarelli.

No one tells a story better in song than Cook, be it the sly commentary of "Makin' Whoopee," or the unabashed sentiment of the little-known "If I Love Again," by Depression-era composer Ben Oakland.

Throughout the evening, Pizzarelli's easy charm and playful musical style complemented Cook's joyful noise, best-expressed in a deeply affecting version of the Shirley Horn standard "Here's To Life" that left Cook and many in the audience verklempt. "Where lucky to live in a world where Barbara Cook lives," Pizzarelli said at the conclusion of that song, and the bravos in the hall echoed that sentiment.

The evening's high point came, though, in the second encore. An acoustic version of John Lennon's "Imagine" with Pizzarelli offering the sparest of guitar accompaniment. It left me thinking Cook can sing anything. Hey, maybe "Beauty and a Beat" next time?

Barbara Cook and John Pizzarelli at Boston's Symphony Hall, April 6, 2013, presented by the Celebrity Series of Boston. For future dates of Barbara Cook, visit her website,