To a critic, the Algonquin Hotel is roughly the equivalent of Cooperstown to a baseball player. Ghosts are omnipresent here: I would never dream of attempting to imitate the urbane iconoclasm of a Dorothy Parker, or, as Robert Benchley would have put it: “drawing on my fine command of the English language, I said nothing.”

American cabaret is an entirely different experience from its counterpart on the Continent, but the evolution of the art form was remarkably similar. In both cases the antecedents are primarily from the serious art-song repertoire that have over time undergone a mutative transformation through contact with the slightly lower-browed classes, resulting, rather paradoxically, in a hybrid of superior sophistication.

The exponents of this arcane genre tend to be the spirits of jazz past (Keely Smith) or present (Diana Krall), or else emerge from the twin colossal arenas of the world of opera, grand (Sylvia McNair) or soap (Andrea Marcovicci). Both these divas are acting out the fantasy of many current opera stars, and anyone who has ever heard Renee Fleming in recital suspects that, in her daydreams, she herself wishes she were at the Oak Room.

The secret of Sylvia McNair’s success may very well be her ability to forget everything that her teachers ever told her. Legitimate opera performers may be at somewhat of a disadvantage in this environment. In order to excel in cabaret, they must unlearn much of what their coaches have ingrained into their psyches. Most especially, they need to loosen up quite a bit. In a very significant opening night of primarily Harold Arlen and Richard Rodgers, Ms. Mc-Nair did just that.

The set was not only entertaining but newsworthy, as this former Pamina and Susanna has bravely announced that she is leaving the operatic stage permanently. She has certainly created a buzz: If the ratio of members of the press to general public was the same at one of her Mozart performances, the Metropolitan Opera House would have had to make room for a thousand critics.

McNair not only recognizes that her crossover is a work in progress, but courageously has fashioned her show to frankly discuss the process. Opening with Albert Hague’s “This Is All Very New to Me,” she immediately established the autobiographical nature of the evening. From the outset, I was impressed with her ability to shape the softest of passages, her sultry and breathy voice allowing for expressive pianissimo.

Dynamics, in fact, were a large part of her presentation. Singing with a microphone, which is normal for this supper-club setting, she worked hard not to overwhelm her audience. But when discussing her prior life on the stage and illustrating with a brief excerpt from Handel’s “Giulio Cesare,” she put down the mic and, in an attention-grabbing bit of synaesthesia, became much louder and fuller of voice. Part of introducing the new Sylvia McNair is reminding us of what we are losing in the shuffle.

She was by far best at the cockeyed optimist type of American song, and so was exceptionally nimble in the Rodgers waltz medley that occupied most of the second half of the program. While her music director, Ted Taylor, kept up a steady three-quarter beat at the piano, Ms. Mc-Nair dazzled with a cornucopia of variations on typical waltz rhythm, from flowing Viennese grace to slightly elongated phrases with a touch of Chopinesque rubato, to down-and-dirty syncopation played off against her accompanist, a technique positively dizzying in the “Falling in Love With Love” section.

Ms. McNair is nothing if not against the grain, concluding the proceedings with Stephen Sondheim’s rebellious “Everybody Says Don’t.” For my money, everybody now says that this is especially intelligent musicianship.

Not that there weren’t some potholes in this new road to freedom of expression. Using Arlen’s “Come Rain or Come Shine” as a throwaway between numbers was positively criminal, particularly since I suspect this fine artist could deliver its romantic intensity as well as or better than any other chanteuse practicing today. And Rodgers’s “Lover” was surprisingly straightforward and metronomic, ignoring the opportunity for boffo accents. Don’t forget; she is a work in progress.

And when Ms. McNair was good, she was very, very good. Her style is disarmingly personal as she totally reinvents herself. Besides the career change, there have also been a dramatic recent weight loss and a major alteration of her domestic situation. She is able to channel it all into her art. It takes a lot of guts to sing much of Arlen’s classic “Over the Rainbow” a cappella, but that is exactly what she did this night. If she has one trait in common with Judy Garland, it is her ability to cut through the blue air to make a direct connection with her audience, projecting unforgettably a quivering, naked vulnerability.

Until January 15 (The Algonquin Hotel, 59 W. 44th Street, between Fifth and Sixth Avenues, 212-840-6800).