Summer McIntosh came within a whisker of breaking a long-standing world record, and Leon Marchand failed to improve on a new mark he set just a night earlier, but both young guns won gold medals at the world championships in Singapore on Thursday.

Romania's David Popovici also flirted with a record on Day 5 at the World Aquatics Championships Arena, with the 20-year-old claiming a thrilling 100-meter freestyle gold to go with the 200 crown he won on Day 3.

The 200 butterfly world record of 2 minutes, 01.81 seconds set by China's Liu Zige in 2009 is the last surviving mark from the supersuit era in women's swimming — and McIntosh all but took it down.

She stormed to her third individual gold medal of the meet in 2:01.99, the second-fastest swim of all time and a yawning three seconds better than American silver medalist Regan Smith.

Having made the last turn under world record pace, the Canadian 18-year-old slowed down slightly coming home and shouted an expletive when she spied the clock.

"I know that I messed up the last 15 meters of my race," she said. "Overall, happy with the time and a PB (personal best), but I didn't reach my goal tonight."

China's 12-year-old marvel Yu Zidi finished just off the podium again having also placed fourth in the 200 individual medley.

McIntosh can console herself in the knowledge that her bid for five individual titles remains intact, with the 400 IM and a hugely anticipated showdown with American great Katie Ledecky in the 800 freestyle still to come.

Only Michael Phelps has won five individual golds at a world championships.

Marchand, dubbed the "French Phelps," celebrated a world record on Wednesday when he blitzed the long-standing 200 IM mark of Ryan Lochte with an incredible swim of 1:52.69 in the semifinals.

With victory seemingly assured in the final, the only suspense was whether Marchand might reset his world record from the previous night, but he ended up nearly a second short, with a time of 1:53.68.

It was still the second-fastest swim ever and led to a dominant win by 0.62 seconds ahead of American Shaine Casas and Hungarian Hubert Kos. Marchand said the record may have taken a bit out of him, but he was hardly complaining.

"I felt so excited yesterday that I couldn’t sleep," he said. "So I think I lost a lot of energy yesterday night, but it was my goal to break the record, so I was really happy with it."

Popovici then emerged as the winner of a sensational men's 100 freestyle final, clocking 46.51 seconds to give Pan Zhanle's world record (46.40) a big scare.

American runner-up Jack Alexy also broke the 47-second barrier (46.92), while Paris Games silver medalist Kyle Chalmers took the bronze for Australia.

At 20, Popovici, the 200 Olympic champion, has already completed the 100-200 sweep twice at the world championships, having done the double in Budapest three years ago.

The relaxed Romanian said he had reached a higher plane; no longer worried about winning or losing.

"I just feel very relieved that this huge pressure of being afraid of winning or losing is off me," he said.

"I don't mean it in an arrogant way, I mean it in a self-maturing way."

The United States celebrated its fourth gold of the meet as Katharine Berkoff won the 50 backstroke in 27.08 seconds, edging compatriot Smith.

In the finale of Thursday's action, Australia's women clinched a second relay gold as Mollie O'Callaghan held off Ledecky in a thrilling final leg to guide her 4x200 freestyle relay team over the finish line.

Australia also won the men's and women's 4x100 freestyle golds on Sunday.