Giovanna Zanerini, whose stage name was Baccelli, was for many years one of the principal ballerinas at the King's Theatre in the Haymarket in London. Her career as a dancer reached a peak in the season of 1780-81 when she appeared in a series of ballets staged by the innovatory French ballet-master J.G. Noverre that took London by storm The costume she is wearing in this picture seems to be adapted from one of these, 'Les Amans Surpris'. From about 1779 Giovanna Baccelli was the mistress of George Sackville, 3rd Duke of Dorset, with whom she lived in lavish style both at his country seat at Knole, Sevenoaks, and in Paris during his term as British Ambassador there from 1783 to 1789. She was painted by Reynolds as well as Gainsborough and there is a life-size nude by Locatelli at Knole.
This is generally agreed to represent one of the peaks of Gainsborough's mature style, displaying particularly his brilliant fluency of brush-work and his ability to create expressive of life and movement. It also exhibits his famed ability to catch a likeness, for which it was praised when shown at the Royal Academy in 1782, although one critic also wittily noted that '... the face of this admirable dancer is paint painted', by which he meant to point out that she is wearing stage make-up (known as paint).
Simon Wilson, Tate Gallery: An Illustrated Companion, Tate Gallery, London, revised edition 1991, p.32