Australian Mack Horton won the men’s Olympic 400 metres freestyle, edging out defending champion Sun Yang of China as Australia set out to erase the memory of their disappointing swimming performance in London four years ago. Gabriele Detti of Italy took bronze.

Horton, 20, was content to shadow Britain’s James Guy and Conor Dwyer of the United States for much of the race but forged ahead after 300 metres, made the final turn in the lead and held off Sun in the last 50 metres.

The 20-year-old is taking part in his first Olympics and will also compete in the 1,500 freestyle and 4x200 relay.

“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,“ he told reporters.

It was a dream start for Australia, a traditional swimming power who faltered badly in London, winning just one gold medal in the women’s 4x100 freestyle relay.

Dwyer, fastest in the morning heats, came fourth, with American Connor Jaeger fifth and Guy, the world silver medallist last year, fading to sixth.

London champion Sun was in tears after the race.

“Based on my ability, I should have been the champion, I trained very hard,” he told reporters.

Sun won two golds at London 2012 and was the first Chinese male swimmer to win an Olympic title.

He has had problems out of the pool since London, serving a three-month ban in 2014 after testing positive for a banned stimulant and spending a week in jail for crashing a car while driving without a licence.

Australia’s women, powered by sisters Cate and Bronte Campbell, beat the United States and Canada to win the Olympic 4x100 metres freestyle relay in world record time, retaining the title they won in London four years ago.

The Australians, leading off with Emma McKeon and Brittany Elmslie swimming the second leg, clocked three minutes, 30.65 seconds, beating the previous mark of 3:30.98 they set in Glasgow in 2014.

Kosuke Hagino won the men’s 400 metres individual medley to give Japan the first swimming gold medal in Rio.

Hagino, the bronze medallist in London in 2012, won with a time of four minutes 6.05 seconds to end a US stranglehold on the event dating back to 1996.

Chase Kalisz of the United States, who had been fastest in the heats, finished second and Japan’s Daiya Seto took the bronze.

Hagino’s time was the third fastest ever and the fastest by a non-American swimmer.

Olympic champion Ryan Lochte of the United States did not qualify for the event at the US trials and nor did world record holder Michael Phelps, the winner in 2004 and 2008.

American Tom Dolan won the event in 1996 and 2000.

Hagino’s win made up for missing out on the world championships last year due to injury, a gold that Seto won in his team-mate’s absence.

Seto led for the first 100 metres, covering the opening 50 metres of the butterfly leg inside world record pace, but Hagino was ahead at the 150 mark and led the rest of the way with Kalisz passing Seto at the end of the breaststroke leg.

Britain's Adam Peaty failed to lower his world record swim in the heats but looks set to stroll to the gold in the 100m breaststoke on Sunday night.

Peaty bettered his own world record, clocking 57.55 seconds in the afternoon.

And he was just seven one-hundredths of a second slower in clocking 57.62secs in his semi-final, more than a second clear of the rest of the field.

Britain won just three medals, none of which were gold, in home waters at London 2012, but there was optimism of an improved showing in Brazil.

And much of the expectation falls on Peaty, who appears a good bet to end a 28-year wait for glory.

Not since Adrian Moorhouse in Seoul, also in the 100m breaststroke, has a British male won Olympic gold.

But Peaty appears unfazed by the pressure and expectation, merely enhancing his credentials as the favourite on Saturday’s opening day of the Games.