Mollie O'Callaghan equals Ian Thorpe with 11 swimming world championship gold medals

Mollie O'Callaghan has equalled swimming great Ian Thorpe's Australian record of 11 world championship gold medals with victory in the women's 4x200m freestyle relay in Singapore.

O'Callaghan anchored the team of Lani Pallister, Jamie Perkins and Brittany Castelluzzo to victory in a thrilling race with the USA finishing second and China in third.

The 21-year-old's teammates laid the perfect platform, handing her a lead going into the final leg of 0.39 of a second over US superstar Katie Ledecky — who was shooting for her 23rd world title to move within three of the record golden haul of compatriot Michael Phelps.

The two had a virtual match-race over the final 200m, but O'Callaghan was never headed to eventually win by 0.66 of a second in an overall time of 7:39.35

The night after winning the 200m freestyle, then swimming second fastest in the semifinals of the 100m, O'Callaghan overcame exhaustion to equal Thorpe's Australian record for world titles in the pool with her third gold of the meet.

"It is very special. It would be wrong if I said it wasn't special. It's a huge accomplishment in a way," said O'Callaghan, who has won four individual and seven relay golds.

"Each moment I can remember, and it just shows the journey that I've had, and us girls have had, because I haven't been able to do all these gold medals without these girls.

"I have a great coach [Dean Boxall] who keeps it really positive – the environment, very passionate, you know he's really outgoing."

O'Callaghan — who on Friday night (AEST) will try to add 100m freestyle gold to her 200m, 4x200m and 4x100m titles at this meet — had to leave the interview early, to calls of "love you", "proud of you" from her teammates.

Pallister, who swam a personal best in her opening leg, said she gave O'Callaghan a pep talk before the final leg.

"Without the profanities, I was pretty much saying to Mollie on the block 'do you know who you are?'," she said.

"When you've had the former world record and you've won the world championship the day before … in my mind as soon as Mollie dives in, I know we've won it."

Pallister and Perkins said the team did not think they would win the final without world record holder Ariarne Titmus, who was in commentary and in the middle of a year off.

"When you lose a 1:52 swimmer and a world record holder, it's really hard to get up and America always lifts for relays, so we knew we were coming in for a battle tonight," Perkins said.

The woman who has essentially replaced Titmus is Castelluzzo, who gave up the lead in her third leg before making up a body length in the final lap to touch in front.

"I was just trying to keep calm and then have fun trying to mow someone down in the last 50m," the 24-year-old said.

"Australia's very good at relays and I did feel a lot of pressure coming in. We've lost Arnie and I'm sort of the one who's come through and replaced [her], so it's big shoes to fill."

Gold medal not enough for Canadian teen superstar

Canadian Summer McIntosh blew the field away to win the women's 200m butterfly final in a time of 2:01.99 – her personal best and the second-fastest time in history.

She was just 0.18 of a second behind the world record set by China's Zige Liu in 2009 during the infamous supersuit world championships in Rome.

As 18-year-old McIntosh turned around to view her time, she let out an audible obscenity in frustration, and it took several minutes before the gold medallist cracked a smile in her post-race interview.

"Winning was the goal going in, but my other goal tonight was to break the world record, which often I don't really say or focus on, but to see how close I was to breaking it and not getting it, I'm a little bit frustrated," McIntosh said.

It was her third victory at these championships as she seeks to win five individual gold medals to equal Phelps's record from Melbourne 2007.

Australian Lizzie Dekkers came third behind McIntosh and American Regan Smith just six weeks after placing third at the Australian trials, which ruled her out of the world championships.

But she was called up when Abbey Connor pulled out for "personal reasons".

"A roller-coaster is really the best way to describe it. It's been a crazy few weeks," she said.

"I'm really stoked with that result. It's definitely not what I expected. I wasn't meant to be here, you know?

"Five weeks ago, I'd hopped back in the water, but I wasn't expecting to race for six months."

She said swimming with McIntosh was inspiring.

"Racing her is amazing, getting the opportunity to internationally race the best every time, it makes me want to push harder, especially because she is so young," the 21-year-old said.

Chalmers's world title defence ends with bronze

Australia's Kyle Chalmers won bronze in the men's 100m freestyle final as Romanian David Popovici swam the second-fastest time in history.

His time of 46.51 seconds was just 0.11 outside the record set at the Olympic Games by China's Zhanle Pan, who missed out on the final.

"I was never going to swim 46.5 or around that mark, unfortunately. I would have loved to see 46 next to my name," said Chalmers, who stopped the clock at 47.17.

"But I'm really, really proud of that performance and to come away with a medal in such a stacked final is something I'm really proud of."

Chalmers said he came close to stepping away from swimming in January, but credited his physiologist and coach for coming up with a new training plan that revived his love for the sport.

"From doing nine sessions a week, 50 kilometres sort of weeks, to now doing once a day topping at probably at 20 kilometres, which is kind of reinventing myself in the sport and finding a new way to swim fast," he said.

"It's been really exciting."

Chalmers was the defending world champion and won silver at last year's Olympics but was almost a second behind Pan, which he said rocked his confidence.

"To be beaten by a whole second in an Olympic final and then try and reset and believe that you're good enough to continue to compete in that event," he said.

"Those guys saw a 46.4 last year and everyone's training and believing they can swim a 46 and 46 has become the new normal."

Chalmers has a new lease on life in and out of the pool. He's engaged to Norwegian swimmer Ingeborg Loyning and the couple are expecting their first child in September, living on a farm in the Adelaide Hills.

"I'm loving doing something so different that's stimulating my brain," he said.

"To be able to just get up in the morning now, have a coffee, do some farm work, kind of go down to training around 11.

"I swim by myself with the public swimmers in the middle of the day.

"Our goal is to make the LA Olympics together."

Frenchman, Leon Marchand could not repeat his world record-breaking pace from Wednesday night, but he still won gold in the men's 200m individual medley in the second-fastest time in history behind only his semifinal swim.

Australia leads the swimming medal table with five gold medals ahead of the US on four, with Canada on three – all thanks to McIntosh.