As a jazz vocalist, Kendra Shank is a free spirit

Over the past 20 years her career has been boosted, and in many ways defined, by the jazz vocal legends who've avidly championed her music, including Jay Clayton, Shirley Horn, and Abbey Lincoln. Shank followed in their footsteps not by borrowing from their styles but by forging a breathtakingly original concept based upon open-ended bandstand interaction.

"There's so much group interplay in the band; when any given person is soloing it can become a group improvisation," said Shank, 51, after a recent performance in San Francisco.

Shank specializes in the kind of intuitive dynamic best created with a working band, and her new album, "Mosaic," marks the 10th anniversary of her capaciously inventive quartet featuring pianist Frank Kimbrough, bassist Dean Johnson, and drummer Tony Moreno. When she makes her first Boston appearance in seven years Wednesday at the Regattabar, Mark Soskin, a frequent Shank collaborator, will be holding down the group's piano chair. She performs with the same personnel on May 16 as part of the Jazz in Rockport concert series.

"Kendra has a great overall concept," says Soskin, a well-traveled veteran whose credits include a 14-year association with Sonny Rollins. "She really affects listeners. They know she's got something that sets her apart, and it's not about scat singing. She allows the musicians a lot of freedom to interpret the music. She likes to take a lot of chances and risks and I love that."

With her clear, fine-grained tone, supremely flexible rhythmic feel, and fearless improvisational sensibility, Shank has honed an expansive repertoire that ranges far outside the American Songbook. Her previous album, 2006's "A Free Spirit," focuses entirely on the songs of Abbey Lincoln, who emerged as a consistently inspired songwriter late in her career.

While many singers have tackled a Lincoln tune or two, Shank delved into her mentor's songbook headfirst, earning Lincoln's praise by remaking the tunes through her rigorous, collaborative group process. Shank is still deeply engaged with Lincoln's material, which often addresses themes other than romance.

"I came to a point where I had lost interest in the standard repertoire, 'I'll die if you leave me, I can't live without you,' kind of stuff," said Shank, who's been based in New York City since 1997. "I've been in and out of relationships enough that I know I won't die if you leave me. I was longing for other things to sing about."

Lincoln's songs also appealed to Shank because the sturdy, deceptively simple melodies and unorthodox structures brought to mind her musical roots. The San Diego-raised singer was a guitar-strumming folkie and French chanson chanteuse living in Paris when she got turned on to Billie Holiday.

Determined to immerse herself in jazz, she moved to Seattle to study with Clayton, an intrepid vocal improviser who has influenced dozens of jazz singers. Clayton insisted that Shank gain command of a standard before using it as a vehicle for improvisation.

"Jay really influenced the direction that I took," Shank said. "On the one hand she said you have to learn a song's melody and the changes. At the same time I was studying free improvisation with her."

Seattle is also where Shank first encountered Shirley Horn. After attending every night of a weeklong engagement, she struck up the courage to introduce herself and ended up making a devoted friend. Several years later, Horn invited Shank to open for her at the Village Vanguard (a New York debut that Miles Davis provided for Horn three decades earlier) and produced her acclaimed first album, 1994's "Afterglow."

While her effusive approach would never be mistaken for Horn's cool, understated style, Shank shares with Horn a confident sensuality that infuses everything she sings.

"I like to approach the music as sound painting," Shank said. "I go for colors and textures, and I look for players who play that way. The drummer's job isn't to keep time. We all keep time. I'm singing as if I'm an instrumentalist when it comes to rhythmic placement and note choices, and at the same time I'm conveying a story."