Díaz made his professional boxing debut at age 16 on June 23, 2000 with a first-round TKO victory over Rafael Ortiz.
On November 22, 2003, Díaz won the WBO youth lightweight title with a sixth-round TKO of journeyman Joel Pérez.
On April 28, 2007, Diaz was upgraded to super champion by the WBA, successfully defending his title against WBO champion Acelino Freitas by TKO after Freitas forfeited the fight before beginning the 9th round, unifying the WBO lightweight championship with the WBA (Super) lightweight title.
Nate Campbell defeated Diaz by split decision to become IBF, WBO, WBA Champion, on March 8, 2008 in Cancún, Mexico.
Diaz became the new IBO lightweight champion in the "No Retreat, No Surrender" main event fight at the Toyota Center in Houston, Texas.
In the opening rounds, Diaz appeared to be winning the fight, pinning Marquez on the ropes and occasionally staggering him with big left hands.
After starting strong in the ninth round, Marquez's precision punching proved to be too much and Diaz was knocked down twice, bringing an end to the fight.
On August 22, 2009, Diaz moved up in weight to challenge Paul Malignaggi for the vacant WBO NABO light welterweight title at a catch-weight of 138½ pounds in his hometown of Houston, Texas.
Díaz maintained a full-time academic schedule while boxing,[1] graduating high school from Contemporary Learning Center in Houston in 2001 [1] and obtaining a bachelor's degree in political science from the University of Houston–Downtown.