In a two-stage elimination math contest participants aged from 10 to 16 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 math contest. More importantly, using the set of participants who take part in both 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 discrimination and differences in reaction to increasing difficulty are ruled out.