Kirani James claimed Grenada's first medal in Olympic history as he stormed to victory in the 400 metres on Monday night.
The 19-year-old, who has been tipped by American legend Michael Johnson to one day challenge his long-standing world record of 43.18 seconds, destroyed the field to win in 43.94.
None of his rivals came close to catching him down the home straight as the world champion crossed the line well ahead of Luguelin Santos, also 19, of the Dominican Republic.
Trinidadian Lalonde Gordon took the bronze while Chris Brown of the Bahamas, who finished fourth at the last Olympics in Beijing, again missed out on a place on the podium.
Meanwhile Belarus' Nadzeya Ostapchuk won the first gold medal of Monday night as she threw 21.36 metres to become Olympic shot put champion.
The 31-year-old took the title from New Zealand's Valerie Adams, who had to settle for silver in 20.70m.
Russian Evgeniia Kolodko took the bronze with 20.48m.
Yuliya Zaripova took gold in the women's 3,000m steeplechase, clocking a new personal best of 9:06.72.
The Russian was an impressive winner, streaking clear down the home straight to cross the line ahead of Tunisia's Habiba Ghribi.
Ethiopian Sofia Assefa took bronze.