She became the lady-in-waiting of Catherine soon after Catherine's arrival in Russia in 1744, and continued to be so after Catherine's elevation to the monarchy in 1762. Catherine called her "Brussja" and entrusted her with her most intimate personal affairs. Catherine said of her that she was "the person to whom I can say everything, without fear of the consequences", and she is described as her closest confidant in her private affairs. Bruce is best known in history as the "l'éprouveuse", the role she played in Catherine's love life. According to legend, Bruce was to "test" the prospective lovers sexually before they became the lovers of Catherine, after they had been suggested by Potemkin, chosen by Catherine, and examined by a doctor. This very same role has then been attributed to her successor as lady-in-waiting, Anna Protasova. This is unconfirmed, and it is unknown how much truth there is in this, but it is a well-known story in history.
Bruce played an important role in developing the relationship between Catherine and Potemkin, notably as a messenger, a role she played until the relationship was consummated in 1773, when it was she who was given the task to persuade Potemkin to leave his exile in a convent and enter into a relationship with Catherine. She is suggested to have been Potemkin's advisor from the beginning in the 1760s.
In 1779, Catherine was directed into a room where she witnessed her latest lover, Ivan Rimsky-Korsakov, having sex with Bruce. The person who directed her is believed to have been Aleksandra von Engelhardt, on the order of Potemkin, who wished for the removal of both Korsakov and Bruce from court. This resulted in the fall of both Korsakov and Bruce: Korsakov was sent in exile to Moscow, where Bruce soon followed him. However, the relationship with Bruce and Korsakov ended and Bruce returned to reside with her spouse. She was dismissed as a lady-in-waiting and replaced by Anna Stepanova Protasova (1745–1826), the cousin of Alexej Orlov, who is mentioned as "l'éprouveuse" in the poems of Byron.
- Simon Sebag-Montefiore : Potemkin och Katarina den stora (2005)
- Marie Tetzlaff : Katarina den stora (1998)