Hoang Xuan Vinh of Vietnam reacts during the awarding ceremony of the Men's 10m Air Pistol Finals of the 2016 Rio Olympic Games at the Olympic Shooting Centre in Rio de Janeiro, Brazil, on Aug 6, 2016.[Photo/Xinhua]

The 41-year-old Hoang Xuan Vinh won Vietnam's first Olympic gold medal with his last shot in the men's 10m air pistol event.

"I'm happy that I can get the first Olympic gold for Vietnam," said Hoang after the final. "It will be my life-time memory."

Vietnam has won two Olympic silvers prior to the 2016 Games, one from weightlifting and the other from taekwondo.

Hoang beat Sao Paulo-born Felipe Almeida Wu of Brazil to second place while China's Pang Wei, Du's husband, took bronze.

Hoang said: "The Brazilian shooter is very fast and strong, but I think only 'try, try, try'. On the last shot I didn't think gold or silver. I think only try."

In swimming, Mack Horton clinched Australia's first gold medal of the Rio Olympics by upsetting favorite China's defending champion Sun Yang in the men's 400m freestyle final.

Horton clocked 3 minutes 41.55 seconds, eclipsing Sun by just 13 hundredths of a second at the Olympic Aquatics stadium. Gabriele Detti of Italy was third in 3:43.49.

Mack, who finished second fastest in qualifying, is competing at his first Olympic Games in Rio. The 20-year-old was the bronze medalist in the 800m freestyle at last year's world championships in Kazan.