In fact, I recall an incident from Sarah Vaughan's biography about that.
Sarah was supposed to sing the finale of some big jazz festival (I have forgotten where... somewhere in Europe I think, tho that's not much help ;-)) with Ella. Beforehand, her manager told her to get Ella into a ballad because if they did a swinger Ella would make mincemeat out of her.
Well, as soon as they got up on stage Ella's trio started a blazingly swinging "How High the Moon." A little into it, Sassy had had enough, and sang out, "Ella, baby, I'm done."
In return, Ella sang out "Sassy, baby, I'm just starting."
Ella made mincemeat out of her.