Maggie MacNeil of London, Ont., knocked off three-time world champion and defending Olympic gold medallist Sarah Sjostrom to win the women's 100-metre butterfly, posting a Canadian-record time of 55.83 seconds at the world aquatics championships on Monday.

Early on, Sjostrom was nearly one second ahead of MacNeil, who was fifth, but the Canadian took charge and caught the reigning Olympic champion with the fastest closing lap — 29.06 — of the eight-woman final and touched first. MacNeil not only handed the Swede her first loss in the event since 2013 but captured Canada's first gold medal at these worlds.

WATCH | Maggie MacNeil wins butterfly gold in Canadian-record time:

Canadian teen Maggie MacNeil posts a Canadian-record time of 55.83 seconds at the world aquatics championships. 2:56

The 19-year-old MacNeil, making her worlds debut on the senior national team, is the second-fastest woman in history after taking down American Dana Vollmer's Americas record of 55.98 from her London 2012 Olympic gold. MacNeil is also just the second female Canadian swimmer to ever win a world title, joining Kylie Masse, who won the 100 backstroke two years ago at worlds.

She was already identified as a future talent but the future has come very quickly. — CBC Sports swim analyst Byron MacDonald on Canada's Maggie MacNeil

"I was very surprised. I couldn't believe it. I looked up and I'm like, 'Oh my God, I'm out of this world right now,'" MacNeil told Swimming Canada.

"I've definitely been working on my second 50. I tend not to go out as fast as the other girls, but I definitely can come back really well."

MacNeil's improvement arc this past year has been nothing short of sensational, according to CBC Sports swim analyst Byron MacDonald.

"A three-second drop in a best time is unheard of. She was already identified as a future talent but the future has come very quickly, indeed," said the University of Toronto swim coach. "She has a devastating kick and her turn is a difference-maker."

MacDonald added the "very intelligent" MacNeil never appears to be affected by the pressure of being on the big stage.

WATCH | MacNeil discusses the path to world gold:

Canadian Maggie MacNeil discusses her victory in the 100m butterfly at aquatics worlds. 0:50

Sjostrom was denied a record fifth title, crossing the line in 56.22, while Emma McKeon of Australia was third on Monday in 56.61. McKeon was second to Sjostrom in the 100 butterfly in the 2017 world final.

MacNeil, who recently completed her freshman year at the University of Michigan, was part of the Canadian women's 4x100 freestyle relay team that won a bronze medal on Sunday at Nambu University Municipal Aquatics Center.

In February, she won individual titles in the 50-yard (46-metre) freestyle (21.65) and 100-yard (91m) fly (49.59) at the Big Ten championships in Bloomington, Ind., before placing second in the 100-yard fly (49.66) at the NCAA championships in March at Austin, Texas.

WATCH | MacNeil helps Canada to relay bronze on Sunday:

Kayla Sanchez, Maggie MacNeil, Penny Oleksiak and Taylor Ruck posted a time of three minutes 31.78 seconds to lead Canada's 4x100 relay team its first medal at the event since 1978. 6:18

MacNeil qualified second for Monday's 100 butterfly final in a personal-best time of 56.52, only 6-100ths of a second off Penny Oleksiak's Canadian mark from her silver-medal winning performance at the 2016 Olympics in Rio. Sjostrom qualified first in 56.29.

MacNeil, others pay tribute to Japanese swimmer

MacNeil's victory on Monday upped Canada's medal total to five in Gwangju, with two silver and two bronze at the two-week event that features swimming, artistic swimming, diving and water polo.

After receiving their medals, MacNeil, Sjostrom and McKeon gathered on the top podium spot and raised their palms to the crowd, displaying a message to ailing 19-year-old Japanese swimmer Rikako Ikee.

"Rikako never give up" it read, with hearts decorating their palms. Sjostrom came up with the idea.

Ikee announced in February that she has leukemia. She was the world junior champion in the 100 fly and had the fastest time in the world last year. She is aiming to return in time to compete in the Tokyo Olympics.

"We're hoping this will show that we're supporting her and we're here if she needs anything," said MacNeil.

WATCH | Rikako Ikee honoured by fellow swimmers:

Gold medallist Maggie MacNeil of Canada and the 2 other medallists of the women's 100m butterfly pay tribute to Rikako Ikee of Japan who revealed her leukemia diagnosis earlier this year. 0:30

Pickrem collects bronze in 200m medley

Canada's Sydney Pickrem challenged for the lead over the last 50 metres of the women's 200 individual medley final on Monday but came up short, placing third in two minutes 8.70 seconds.

WATCH | Sydney Pickrem charges late to reach podium:

Canada's Syndey Pickrem finishes 3rd at the world aquatics championships with a time of 2 minutes 8.70 seconds. 3:56

Katinka Hosszu, the unbeatable Hungarian, prevailed in a 2019 world-leading time of 2:07.53 for her fourth consecutive gold medal in the 200 IM at worlds. The 2016 Olympic gold medallist is also the three-time defending world champion in the 400 IM. Ye Shiwen of China rounded out the podium on Monday in 2:08.60.

"It's an honour to be on the podium with two girls who are Olympic gold medallists, so they definitely inspire me and get me to push myself," Pickrem said.

"Tonight has to be one of the greatest nights ever if not the greatest night for a Canadian swimming team at a world championships," said Canadian coach John Atkinson. "Our athletes continue to convert opportunities."

"It might seem just another gold medal but for me it's really special to be here and be able to win," said Hosszu, who last year filed for divorce from her husband and training partner. "It's been a tough journey."

The 22-year-old Pickrem, a dual Canadian/American citizen, shone at the recent FINA Champions Swim Series in Indianapolis, finishing second in the 200 medley. Her 2:08.61 put her just behind Hosszu (2:08.50) and ahead of Melanie Margalis (2:10.41) of the United States.

Japan's Yui Ohashi, who was considered a medal contender on Monday, was disqualified from the race.

Masse top qualifier for 100 backstroke final

Kylie Masse, the reigning world champion in the 100 backstroke, qualified first for Tuesday's final in 58.50 seconds. The native of LaSalle, Ont., won world gold in 2017 with a then-world record time of 58.10, breaking a mark that had stood for eight years.

But Masse's time had a much shorter shelf life as American Kathleen Baker swam 58-flat at the U.S. swimming championships last July.

Taylor Ruck of Kelowna, B.C., was third in qualifying Monday in 58.83 while Baker was fourth in 59.03.

"With Kylie and Taylor through to the final of the women's 100 back in great lanes for the final tomorrow night it was really job done tonight for the team," Atkinson said. "Two days of eight down and now it's time to keep focused and on track for the next six days."

In another Canadian result, Vancouver resident Markus Thormeyer placed 11th in the men's 100 backstroke semifinal in 53.59.

Peaty captures men's breaststroke title

Adam Peaty on Monday became the first man to win a third 100-metre breaststroke title at worlds.

The British swimmer claimed the title in 57.14 seconds, a night after he became the first man to break 57 seconds in the semifinals. Peaty was under his own world-record pace at the turn before coming home a full body-length in front and 1.32 seconds ahead of teammate James Wilby.

In the semifinals, the 2016 Olympic champion was timed in 56.88. Wilby touched in 58.46 and Yan Zibei of China was third in 58.63.

"That'll fuel me for next year because I know how bad I want to clear 56 even faster now," Peaty said. "I know exactly how to do it but I've obviously run out of opportunities here."

Horton given warning for podium protest

China's Sun Yang was back in the pool for the 200 freestyle semifinals a night after winning the 400 free. He qualified second-fastest behind Clyde Lewis of Australia. The final is Tuesday night.

Earlier Monday, FINA's executive board met in Gwangju to discuss Mack Horton's podium protest against Sun and decided to send a warning letter to Swimming Australia and to Horton.

Australian Mack Horton refused to stand next to Chinese swimmer Sun Yang on the men’s 400-metre freestyle podium. Sun is currently facing allegations of doping rule violations that could result in a ban from the Tokyo 2020 Olympic Games. 0:59

"While FINA respects the principle of freedom of speech, it has to be conducted in the right context," the board said in a statement.

Horton refused to take his spot on the medals stand or shake Sun's hand after finishing second to the Chinese star in the 400 free. The Aussie swimmer is angry that Sun, who served a three-month doping suspension in 2014, is being allowed to compete in Gwangju before he faces a Court of Arbitration for Sport hearing in September that could potentially end his career.