In two-stage elimination math contests participants from four different age groups compete to pass from stage 1 to stage 2 and later to be among the winners. Although female participants have higher Math grades at school the gender gap reverses in the two stages of the contests. More importantly, following the same individual participant across different stages, we find that the gender gap in performance increases from stage 1 to stage 2 of the competition. The increase in female underperformance is attributed to higher competitive pressure and alternative explanations based on selection, discrimination and differences in reaction to increasing difficulty are ruled out.