FUKUOKA, Japan, July 26, 2023 (BSS/AFP) - 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 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 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 teammate 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 men's 4x100m freestyle relay and 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, nighttime I'm always a whole lot faster, so it would be good to put myself in one of those middle lanes for tomorrow so I can be a part of the race."

- Marchand eases through -

Chalmers's Australian teammate Kaylee McKeown also safely negotiated her heat in the women's 50m backstroke.

McKeown, who won the 100m backstroke title the previous night, finished fourth-fastest with a time of 27.60sec.

American Regan Smith, who took silver behind McKeown in the 100m, was quickest on 27.31, followed by Canada's Kylie Masse on 27.48 and American Katharine Berkoff on 27.56.

French superstar Leon Marchand eased through his heat in the men's 200m individual medley.

Marchand, who broke American great Michael Phelps's 400m IM world record on the competition's opening night, was ninth-fastest with a time of 1min, 58.38sec.

He is set to compete in the 200m butterfly final later on Wednesday.

Britain's Duncan Scott went through as the fastest qualifier on 1:57.76, followed by Japan's Daiya Seto on 1:57.80 and So Ogata on 1:57.88.

Canadian teen Summer McIntosh progressed from her women's 200m butterfly heat.

McIntosh, the defending champion, was third-fastest in a time of 2min, 07.91sec.

Denmark's Helena Rosendahl Back was the top qualifier on 2:07.57, followed by Australia's Elizabeth Dekkers on 2:07.71.