The 18-year-old raced a sizzling final 50m to touch in 3:43.36 and overhaul Australian Jack McLoughlin (3:43.52) and US swimmer Kieran Smith (3:43.94), who took bronze.

It was a major breakthrough for the young gun, who has little pedigree on the world stage.
"I just can't believe that, it's amazing. I felt better in the water this morning than yesterday and that's it. I'm the Olympic champion now," he said."I just put my head in the water and that's it. I just can't believe it. It's a dream come true."
Elijah Winnington, who shocked Mack Horton at the Australian trials to deny his countryman the chance of defending his Olympic title, led through 100m.
But McLoughlin hauled him in by the 200m mark and looked destined for gold until Hafnaoui's burst of electric speed as he zeroed in on the finish line. American Kalisz wins Olympic 400m individual medley gold

Rio Olympic runner-up Chase Kalisz went one better to claim gold in the men's 400m individual medley for the United States Sunday to kick off the medal rush in the Olympic pool.

Kalisz led by a body-length after 300 metres and came storming home in 4mins 09.42secs ahead of second-placed compatriot Jay Litherland (4:10.28) and Australia's Brendon Smith in third (4:10.38)
"It means the world. This is the last thing I really wanted to accomplish in my swimming career," said Kalisz.

"It was a dream of mine since as long as I can remember. I can't believe it. I really can't believe it."
France's Leon Marchand set off at a cracking pace and turned first after the butterfly leg, but Kalisz was always in touch and after a strong backstroke swim took charge in the breaststroke.

It turned into a mad scramble for the minor placings with Litherland just bettering Smith.

Kalisz took silver behind Kosuke Hagino at Rio in 2016, but the Japanese opted out of the event on home soil to focus on the 200m medley.

Rio bronze medallist and world champion Daiya Seto failed to qualify for the final.