Australia's Kyle Chalmers said Wednesday that "no one's invincible" after watching superstar Romanian rival David Popovici flop badly at swimming's world championships the previous night.

Popovici is one of the sport's brightest stars and came into the competition in Fukuoka as the defending champion in the men's 100m and 200m freestyle.

The 18-year-old looked set to retain his title in the 200m final on Tuesday night, powering into a commanding lead heading into the home straight.

But he faded badly over the final 50 metres and ended up out of the medals completely, touching the wall in fourth place.

Chalmers is one of Popovici's main rivals for the 100m title and he said he "would be lying if I said I didn't" take encouragement from watching the Romanian's crown slip.

"No one's invincible -- I've been in this sport a long time and I've learned that over my years," said Chalmers, who qualified from his 100m heat with a third-fastest time of 47.71sec.

"That's a lesson he's probably starting to learn now."

Popovici also qualified from the 100m heats, with a sixth-fastest time of 47.90.

Britain's Matthew Richards, who took gold in the 200m freestyle, had the fastest time with 47.59, followed by American Jack Alexy on 47.68.

Chalmers is back at the world championships having stepped away from swimming for a period last year.

It followed intense media attention on a supposed rift with former girlfriend Emma McKeon and team-mate Cody Simpson, who are now a couple.

Chalmers won 100m freestyle gold at the 2016 Rio Olympics and silver at the pandemic-postponed Tokyo Games five years later.

He has already bagged a title in Fukuoka in the 4x100m freestyle relay and he is hoping for more success in the 100m freestyle semi-final later on Wednesday.

"I think it's my fastest heat I've ever done so I'm happy with that," he said.

"Obviously night time I'm always a whole lot faster so it will be good to put myself in one of those middle lanes for tomorrow so I can be a part of the race."

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