At the 2004 Summer Olympics in Athens, Diaz teamed up with Joshua Ilika Brenner, Alejandro Siqueiros, and Leonardo Salinas Saldana in the men's 4 × 200 m freestyle relay. Swimming the third leg, Diaz recorded a split of 1:53.34, and the Mexicans rounded out an eight-team field to last place and fifteenth overall in a new national record of 7:29.54.