One day in July, Hélène Grimaud was practicing the piano in a hotel room in Munich. The Palace, where she was staying, is near the Prinzregententheater, and is unusually accommodating of classical musicians; Room 606 comes equipped with a Steinway. Grimaud needed to work on the piano part to Mozart’s concert aria “Ch’io mi scordi di te,” which she was scheduled to record the next day, for Deutsche Grammophon. The instrument, an old upright, was not well tuned, but that did not bother Grimaud, who does not fetishize refinement. Offstage, Grimaud, who is small, often resorts to standing on the balls of her feet. But at the piano her powerful shoulders and muscled forearms make an impression. She goes at the music with flat-fingered stabs, her body crouched over the keyboard, like a swimmer preparing to dive.

Grimaud’s room was mostly glass, and she likes musical extremes, so the place soon vibrated with sound, augmented by the grunting chant with which she marked passages in the aria. In “Ch’io mi scordi,” the piano plays faithless lover to the brokenhearted soprano. Soprano: I can’t bear to leave—I’ll stay with you forever, whether you want me to or not. In response, the piano teases, flirts, revels in being desired. Grimaud, who once told the Times that she should have been born a boy, played the exchange with a taunting lilt—La-da deeh-da dah-da-dah. Grunt. Da-dah-da dah-da la-da-dah. Grunt. By the time she got to the rolling arpeggios of the song’s resolution, you felt certain that the soprano had chosen the wrong shoulder to cry on.

Grimaud doesn’t sound like most pianists: she is a rubato artist, a reinventor of phrasings, a taker of chances. “A wrong note that is played out of élan, you hear it differently than one that is played out of fear,” she says. She admires “the more extreme players . . . people who wouldn’t be afraid to play their conception to the end.” Her two overriding characteristics are independence ...