The potential shift in Brazil s energy pricing paradigm from the current Audited Costs model to the Bid-Bases model, with the aim of modernizing the electricity sector and seeking practices that encourage competition, implies the need for preliminary studies to assist the transition process and define the appropriate market design for the Brazilian reality. In this sense, the use of equilibrium models, notably the Nash Equilibrium, emerges as a powerful ex-ante tool that allows the analysis of competitors behavior to identify possible inefficiencies to be mitigated.To this end, this thesis models the optimal bidding decision process of a competitor in a hydrothermal day-ahead electricity market using a bilevel optimization model, enabling the identification of the Nash Equilibrium of the market through an algorithm based on the Gauss-Seidel. Additionally, the method is applied to two numerical experiments: a three-bus test system and a representative case of the complete Brazilian system, allowing for the analysis of competitors behavior by comparing the results with the Audited Costsand Competitive Equilibrium models. The impacts of water inflows and hydro plants in cascade with distinct ownership on revenues and consequently competitors behavior were observed. Finally, the occurrence of competition exclusively in quantities was verified, as well as the practice of quantity bids retention by competitors in order to modify the market equilibrium price, increasing their revenues.