The last time Kyle Chalmers dived into the water at the OCBC Aquatic Centre, he was competing in last year's Fina World Junior Championships.

Last night, the Australian stood on the starting blocks at the same venue, ready for another race - except now he carries the tag of an Olympic champion, after stunning the world with his 100m freestyle win at the Rio de Janeiro Games.

The 18-year-old did not win yesterday in the Fina/airweave Swimming World Cup, a short-course meet, finishing second to Russia's Vladimir Morozov in the 100m free.

"I'm not a good short-course swimmer, but to have a new personal best in my first couple of races since Rio is pretty promising," he said.

Reflecting on his amazing rise, Chalmers told The Straits Times: "Twelve months ago I was over here racing and I was just happy to come away with three golds, three silvers and a bronze. To win an Olympic medal eight months down the track was something I never thought was achievable."

Yet he managed that, in a 47.58sec victory that was jaw-dropping in more ways than one.

Aside from being the youngest in the pool that day, he was also second-last with 50m left in the race, before becoming the first Australian since 1968 to win the event.

Interestingly, what helped him was not knowing who he was up against. "The thing I think I had to my advantage was that I don't know a whole lot of swimmers," he said.

"I'm trying to remember who was even in the final... I hadn't met a whole lot of them (before Rio) but it's really cool knowing them now."

For the record, he managed to name five of his rival swimmers in the eight-man final.

Even as a country with a storied swimming history pins its hopes on a new poster boy, he is unfazed by it all.

"It's easy to stay grounded because I'm a country boy," said Chalmers, who hails from Port Lincoln, a town of some 16,000 people in South Australia.

"Growing up in the country made me I guess very relaxed with a lot of things. It doesn't get to me."

Indeed, when he spoke to The Straits Times during Friday's press conference, it was with the same respectful demeanour - hands-behind-the-back, warm smile, inquiring eyes - although this time there was a shyness when talking about his gold medal.

Things are certainly different now. For instance, he cannot walk down a street uninterrupted in Adelaide, where he trains.

His Instagram followers have increased from around 6,000 pre-Rio to 45,000. He was even ambushed by fans while on holiday in London, where he watched his favourite football team, Arsenal, in action.

But it doesn't detract from his ultimate aim: "My biggest goal is defending my title in Tokyo and I'm going to do everything I can to make sure I achieve that."

Hungary's triple Olympic champion, Katinka Hosszu, was also in action last night, breaking the 200-gold mark since she began competing in the Swimming World Cup in 2012.

Her double century of wins came when she took out the 100m butterfly, after winning the 200m freestyle. Later in the evening, she clinched the 200m individual medley and 200m backstroke golds too, taking her overall tally to 202 - for the time being.

Said Hosszu: "These numbers are pretty amazing. I'm still excited coming every day into the finals session. I'm going to try and add a couple more."