I did a first year on a PhD Economics course where we taught modules so I hope I can answer.
Yes Economics = Maths at the highest level (after BSc)
Be very sound in these areas:
1) Optimisation (Lagrangian, Simplex, Runge Kutta)
2) Statistics ( OLS, GLS, IV, GMM, Time Series, Central Limit Theorem, Hypothesis Testing)
3) Calculus (Multivariable as well as normal)