I saw her sing live years ago. She's probably one of the greatest singers in our lifetime; but, since she doesn't sing that bullshit like most singers do, most people don't know who she is today.
I'd say she'd sing circles around almost any singer a person can name (in their prime) with the exception of Chaka Khan. She along with Luther Vandross, and Michael McDonald sung background for Steely Dan. Steely Dan only used "THEE" best.
She was performing with Lee Ritenour, Jonathan Butler, Gerald Albright, and several others. It was a helluva show.