Masso was born in Brussels, Belgium to an Argentine-born father, Eduardo, and a Belgian mother, Sabrina Merckx. He comes from a family of sportsmen. His father Eduardo Masso is a former tennis player who represented Belgium at the Davis Cup. His maternal grandfather is the Belgian former cycling champion Eddy Merckx, a five-time Tour de France winner. His maternal uncle, Axel Merckx, is an Olympic bronze medalist in cycling for Belgium.