This paper proposes strategic biddings for a consumer demand that participates in both the day-ahead and balancing markets. The strategic behavior of the consumer is represented by the bilevel optimization programming with minimization of consumer costs at the upper level (UL) subject to the co-optimization of energy and reserve in the market clearing process at the lower level problem (LL). Using the Karush-Kuhn-Tucker (KKT) optimality constraints to replace the LL problem, the bilevel model is recast into a single-level mathematical program with equilibrium constraints (MPEC). The resulted model is finally formulated as a mixed-integer linear programming (MILP) problem using the exact linearization technique and Fortuny-Amat transformation to replace bilinear terms and the complementarity constraints, respectively. The results demonstrate a reduction in the electricity consumption payment for the strategic consumer and a decline in social welfare.