The 19-year-old Diaz pulled away from his rugged opponent Pavlo Ishchenko of the Ukraine, landing uppercuts and straight right hands in the final round as he skated to an impressive 19-9 victory.
Diaz faces a tough task to nudge towards the medal rounds, but said: 'The Cuban beat me last time we fought but I felt I could have done a little more. I've prepared well and that's what I'm going to do.'
Also at Bantam Weight, Australian teenager Ibrahim Balla survived an all-action showdown against Morocco's Aboubakr Lbida, crucially taking the final round to claim victory on countback after a 16-16 draw.
Former world champion Detelin Dalakliev of Bulgaria easily beat South Africa's Ayabonga Sonjica 15-6, while Italy's Jayn Parrinello will be Luke Campbell's first opponent after he beat Namibia's Jonas Matheus 18-7.
But there was heartbreak for 17-year-old Londoner Isaac Dogboe, representing Ghana, who took a two-point lead into the last round against Japan's Satoshi Shimizu but ended up on the wrong end of a 10-9 verdict.
At Middle Weight, American Terrell Gausha staged a spectacular fightback against Armenia's Andranik Hakobyan, flooring his opponent twice in the final 10 seconds to force a stoppage with no seconds left on the clock.
In a wild, swinging contest, Hakobyan got the better of the first two rounds to lead by a point heading into the third. A huge overhand right decked the Armenian and a big follow-up shot gave Gausha the dramatic victory.
Uzbekistan's powerful former two-time world champion Abbos Atoev beat Badr-Eddine Haddioui of Morocco 11-9, while Azerbaijan's Soltan Migitinov looked a possible contender in defeating Egypt's Mohamed Hikal 20-12.
Cheered by a sizeable contingent of his compatriots, Indian superstar Vijender nudged into the last 16 at the expense of Kazakhstan's Danabek Suzhanov, extending his lead throughout to take a 14-10 verdict.
Great Britain's Anthony Ogogo produced a strong and focused performance against Junior Castillo from the Dominican Republic, easing away to score a 13-6 victory and earn a place in the last 16 of the Middle Weight competition.