When Renée Fleming met her idol Joni Mitchell for the first time not long ago, she was faced with two choices. "I could fall at her feet and grovel, sobbing hysterically, or I could say, 'How nice to meet you,' " she recalled from the stage of Joe's Pub on Thursday evening. She ended up making the demure choice.

But to hear her plunge fearlessly into the turbulent rapids of Ms. Mitchell's song "River" at the first of two shows on Thursday evening was the concert equivalent of observing a great classical singer worship abjectly at the feet of a pop composer. Refusing to put a safe distance between herself and the song (from Ms. Mitchell's album "Blue"), she held nothing back; her operatic intensity was matched by an emotional immodesty that was the furthest thing from demure. As she sobbed, "I made my baby cry," Ms. Fleming and the song became one.

"River" is one of several remarkable cuts on Ms. Fleming's recently released album, "Haunted Heart" (Decca), in which she sings a mostly pop program, accompanied by Fred Hersch on piano and Bill Frisell on guitar. At Joe's Pub, at the Public Theater, where she performed as a benefit for the club, she was joined only by Mr. Hersch, a jazz piano master who shares both her classical roots and her passion for serious songs that might be called high pop.

Together Ms. Fleming and Mr. Hersch made a persuasive case that the best popular songs, whether by Cole Porter or Ms. Mitchell, can be negotiated by a classically trained singer with the right adjustment of technique and point of view. The first rule must be to avoid any suggestion of condescension; hauteur is out. There is plenty of latitude. Dawn Upshaw succeeds with a light approach, Ms. Fleming with a heavy one.

A provocative illustration of the proximity of genres was the segueing of an instrumental excerpt from Berg's opera "Wozzeck," played by Mr. Hersch, into the song "Midnight Sun." As one slid into the other, Ms. Fleming rolled her voice around Johnny Mercer's lyrics with an intoxicated delight that recalled Sarah Vaughan's voluptuous romp through the same words (minus Vaughan's inner swing).

Best of all were the Dietz-Schwartz ballad "Haunted Heart," which Mr. Hersch's lingering arrangement and Ms. Fleming's anguished vocal took to a very dark place, and "You've Changed" (lyrics by Bill Carey and music by Carl Fischer), another torch song, which the singer cited as her personal favorite.

But the chemistry between Ms. Fleming and her material was far from infallible. "My Cherie Amour," performed without a studio arrangement and without the vocal lilt of the young Stevie Wonder to lift it, came across as fluff. At the opposite pole, the Beatles' "In My Life," attenuated into a lugubrious illustration of "an 11 o'clock number," as Ms. Fleming called it, sank under the weight. Like so many pop songs, "In My Life" must be allowed to catch you off guard, its sadness permitted to sneak through a veneer of charm.