>Turandot, the opera Nessun Dorma
She doesn't guess his name, she lies about it.
The Prince, however, then reveals his name: "Calaf, son of Timur – Calaf, figlio di Timur", thereby placing his life in Turandot's hands. She can now destroy him if she wants (Duet – Turandot, Calaf: "Del primo pianto").
BTW Timur is Tamerlane, or Timur the Lame