CAMBRIDGE -- Fred Hersch has been hailed as ''a poet of a pianist" by no less an authority than The New Yorker's longtime critic Whitney Balliett. Though Hersch was influenced by such seminal jazzmen as Billy Strayhorn and Thelonious Monk, the jazziest thing about him is his firm commitment to improvisation. His playing bears traces of everything from classical music to folk-rock, and he has composed a song cycle based on Walt Whitman's ''Leaves of Grass."
Friday night, at the Regattabar, Hersch opened with a medley honoring two lesser-known forebears: Chet Baker's pianist Russ Freeman and undersung songwriter and composer Alec Wilder. His tender traversal of Freeman's ''The Elements" began with quiet plinks and progressed to out-of-tempo whirls and eddies before easing into wispy melody. He segued to Wilder's ''Moon and Sand," and his playing darkened and became more propulsive, with increasingly daring right-hand figures. Yet rather than swinging in typical jazz fashion, Hersch's rhythms surged like the ocean.
Up next was Monk's ''Played Twice." Though Hersch has clearly learned much from the master, he rarely apes Monk's mannerisms. Instead, he applies Monk's gracefully awkward timing and inflected silences in his own gentler way. His charming, herky-jerky rendition of the tune explored the full range of the piano, from rumbling bass to tinkly treble.
''At the Close of Day," one of Hersch's Whitman settings, evoked Bill Evans with its moon-dappled melancholy. He took Fats Waller's ''I'm Crazy 'Bout My Baby" for a rewardingly risk-taking ride, though his rhythms seemed less stride than stumble. His version of Hoagy Carmichael's ''The Nearness of You" displayed the sensitivity that makes him a sought-after accompanist.
The unexpected treasure of the set was Hersch's spare, almost sculptural reading of Joni Mitchell's ''My Old Man." He dug into the tune rather than decorating it, elaborating its eccentric jazzy turns with mysterious bass figures and quiet, spiky dissonances, making every note count.