Sun Yang jumps into the water in the men's 400 meters freestyle final.[Photo/Agencies]

Sun was the pre-race favorite, having won gold in the same event at the London 2012 Games. The 24-year-old is also the defending 1500m Olympic champion.

Hungary's Katinka Hosszu set a new world record to win the women's 400 meters individual medley. The 27-year-old finished in four minutes 26.36 seconds to beat the previous mark of 4:28.43 set by China's Ye Shiwen at the London 2012 Games.

Maya Dirado of the United States was second in 4:31.15 and Spain's Mireia Belmonte third in 4:32.39.

It was Hosszu's first podium finish at the Games, despite having netted nine world championship medals - five of them gold. Hosszu is making her fourth appearance at an Olympic Games in Rio.

Australia took the second gold in the pool as they won women's 4x100m freestyle relay in new world record.

Kosuke Hagino won Japan's first gold medal by beating American Chase Kalisz and countryman Daiya Seto in the men's 400m individual medley.

Earlier in the day, Britain's Adam Peaty broke his own world record for the men's 100m breaststroke in the heats.

In judo, Paula Pareto from Argentina and Beslan Mudranov from Russia took the women's 48kg and men's 60kg titles apiece.

The men's cycling road gold went to Belgium cyclist Van Avermaet Greg while Emese Szasz earned Hungary its first gold medal in Rio with victory in the women's epee individual event.

South Korean archers once again showed their overwhelming dominance as they outclassed a consistent American team 6-0 in the final.

Elsewhere, Sopita Tanasan of Thailand won the gold medal of the women's 48kg contention with a total of 200kg.