Midway through opening night at the Kennedy Center's Terrace Theater, Nacha Guevara slowly dropped to her knees, back turned to the audience, and positioned herself in front of a performer's traveling trunk. Lifting the cover revealed a gigantic mirror and Guevara sang "Send In the Clowns" -- with less poetic, more sincere Spanish lyrics -- in such a way that each member of the audience had to feel like an intruder in her utterly private thoughts. Stephen Sondheim's song must be one of the most overworked of the last quarter-century, yet last night the Argentine cabaret-diva transformed it into a moment of elegant poignance.

For Jacques Brel's elegiac "Old Folks," Guevara sat on the lip of the stage, a metronome beating as slowly as a clock winding down while husband Alberto Favero wove empathetic piano phrases around Brel's graceful introspections (sung in English, as were 14 of the show's 20 songs). For most of the night Guevara stalked the stage with the confidence of a model and the tautness of an actress, establishing and then reinforcing the physical and emotional paramaters of her songs. Her supple face, with huge eyes pinched by high, tense cheekbones, performs its own wrenching choreography of emotions.

Guevara's as much actress as singer, partly because her voice is not remarkable; like its owner, it's a bit thin but often redeemed by a deep emotional investment in the material. Earlier, the lithe Guevara had stood stock still, a sardonic Dietrich coldly embracing the increasingly frenzied lyrics to "Hate Song," a send-up from the mid-'60s "Mad Show." For "My Man," a sprightly song of possession from a '20s French revue, she enveloped herself in a costume that turned her into a soft sculptured fat-lady who laced the lyrics with Chaplinesque counterpoint. There were a dozen distinct personalities in between.

To satisfy American audiences, Guevara now performs mostly in English; she is, at best, uneven, with her body English more assured than her Berlitz. On slower material -- the Brel, a languid reading of Shakespeare's "Sonnet No. 30," a disquieting "Not While I'm Around" from Sondheim's "Sweeney Todd" -- Guevara has all the necessary control. That's less the case when the lyric lines are crowded or when the tempi quicken, as on "Welcome to the Theatre" or "Nostalgia," or Noel Coward's raggedly witty "Why Must the Show Go On?", a challenge even for a singer proficient with his language; then, too many lyrics slip by.

The shame is that Guevara doesn't sing more in her native Spanish because it is then that she becomes a presence on the order of Piaf or Garland. There's a three-song passage that encapsulates the earthy, purist Latin American populism expressed in the poems of Mario Benedetti and Jose' Marti'. Marti''s "Versos Sencillos" ("Sentimental Verses") insists that the only real tragedy in life is when people are enslaved, while Benedetti's "Te Quiero" explores the terrain of the heart in haunting images. Guevera sings the latter twice, first in English with a Piaf-like soft-spoken urgency and then in Spanish without a hint of reserve. It points up the layer of self-consciousness that exists when the lyrics are a step removed from the heart. A third pokes fun at someone who seems to enjoy their misery. Although Guevara sang these standing still, the internalized energy that simmered elsewhere burned intensely, transcending any need for translation.

Working on a gothically simple set that suggested more than it defined, and beautifully lit (not by follow spots but by top-lighting), Guevara ended with a quietly urgent Spanish lyric to "Don't Cry For Me, Argentina" that was as much a heart-stopper as a show-stopper. Like a Colorado colt, her voice ran free on a range of emotions that left the audience drained. Standing like an unsuited Pierrot with a bulls-eye heart, Guevara sang both about herself and her country with shattering directness: as the embattled Evita responding to her doubters, she sang "look in my eyes that are crying with love." Teardrops echoed in the stilled Terrace Theater.

NACHA. With Nacha Guevara. Alberto Favero, musical director; William Ivey Long, art director; Duke Durfee, scenic designer; Richard Winkler, lighting designer; Norman Briski, artistic consultant.

At the Terrace Theater through Nov. 14.