Altamirano won the 2013 USTA Boys 18 National Championship as an unseeded player, the first ever to do so, defeating Jared Donaldson 6–1, 6–2, 6–4 in the final. This earned him a wild card entry into the 2013 U.S. Open where he lost to 22-seed Philipp Kohlschreiber of Germany in the first round. Prior to the 2013 U.S. Open, Altamirano had only played in six professional-level matches, all at the ITF Men's Circuit level (commonly referred to as Futures), and had lost all six.
