Mack Horton has delivered Australia its first gold medal of the Rio Olympics with victory in the men's 400 metres freestyle final.Horton, ranked number one in the world heading into Rio, swam his race to perfection to out-touch China's defending champion Sun Yang 3:41.55 to 3:41.68.
It was Australia's second medal of day one, after the men's archery team won bronze. Australia's women's 4x100m freestyle relay team then topped off a golden night in the pool.
Horton's triumph gave Australia its sixth gold medal in the event at the Olympics, meaning Horton joins the likes of Ian Thorpe (2000, 2004) Murray Rose (1956, 1960) and Brad Cooper (1972).
It was a personal best for the 20-year-old and he sits behind only Thorpe on the Australian all-time list.
"In another big final, it's about touching the wall first and that's what I did. I had my eye on everyone else in the field," he said.
"It hasn't really sunk in yet. I have a couple more races this week so I need to relax a little bit and focus on those but it's very exciting."
There had been bad blood brewing between Horton and Yang in the build-up to the final, after the Chinese had splashed water in the face of the Victorian during a training session at the Olympic aquatic centre earlier in the week.
Horton did not take it too seriously, although he did make a reference to Yang's doping history following the heats on Saturday (AEST), setting up a grudge match of sorts in the final.
Neither Horton nor Yang set the early pace, however, as it was Great Britain's James Guy he led the field through the opening 200 metres under world-record time.
Horton was content to settle into his rhythm and from the halfway point of the race to the 300-metre mark he began to make his move, shifting from third place to second.
Guy still led with 100 metres remaining before fading in the penultimate lap, as Horton changed gears and headed to the front with Yang at his heels.
Horton held a narrow 0.14-second lead over Yang heading into the final 50 metres, but was able to the hold off the two-time gold medallist in the sprint to the wall.
Italy's Gabriele Detti won the bronze medal in 3:43.49, while Horton's compatriot David McKeon was seventh in 3:45.28.
Horton will have a chance to push for another individual gold medal, with the 1,500m freestyle still to come on his program in Rio.
In the other men's final contested, Australia's Thomas Fraser-Holmes and Travis Mahoney finished sixth and seventh respectively in the men's 400m individual medley.
Fraser-Holmes touched the wall in 4:11.90 and Mahoney clocked 4:15.48.
Japan won gold and bronze, with Kosuke Hagino (4:06.05) and two-time world champion Daiya Seto (4:09.71) making the podium.
American Chase Kalisz, the fastest qualifier from the heats, split the Japanese duo to take silver in 4:06.75.
Another of the Australian men competing on night one was Jake Packard, who missed the final by one spot after finishing fourth in his semi-final with via a time of 59.48.
Great Britain's Adam Peaty led qualifying with a time of 57.62, just .07 outside the world record he set in the heats.