Hangzhou - Japan's Daiya Seto smashed Chad le Clos's short-course record in the men's 200m butterfly as he grabbed gold at the world championships to stun the South African on Tuesday.
The 24-year-old Seto pipped his more famous rival by only 0.08 to make a piece of history on the first day of action in the Chinese city of Hangzhou.
Seto, who won bronze in the 400m medley at the Rio 2016 Olympics, just about held off Le Clos down the stretch to triumph in 1:48.24.
Le Clos also beat his previous world-leading time of 1:48.56, set in Singapore in 2013, but the reigning long-course and short-course champion had to settle for silver.
A distant third was China's Li Zhuhao.
"I am so happy and surprised that I set a world record. I wake up every day wanting to set a new record and I finally did it, it is amazing," said Seto.
Le Clos, 26, swimming out of lane one, said: "It stings to lose like that.
"I broke the world record tonight and the national record, and I got silver. I don't know how that happened."
Also record-breakers were the United States, who bettered their own nine-year world mark to surge to gold in the men's 4x100m freestyle.
Powered on the first leg by Caeleb Dressel, the brilliant 22-year-old who has drawn comparisons with US swim legend Michael Phelps, the Americans romped home ahead of the Russians in a record time of 3:03.03.
That trumped their December 2009 short-course record of 3:03.30.
Australian teenager Ariarne Titmus stormed back to win the women's 200m freestyle gold and underline her status as a rising star of swimming.
The 18-year-old, who won 400m and 800m freestyle gold at the Commonwealth Games in her home country earlier this year, ran down the American Mallory Comerford to seal victory in a thrilling race.
Titmus was back in third after 150m, but she burst to the finish in a time of 1:51.38 to relegate Comerford into silver, with Femke Heemskerk of the Netherlands taking bronze having led for half of a nail-biting race.
Titmus put her head in her hands when she realised she had taken gold, powering to the win with 0.43 to spare over Comerford.
Hungary's Katinka Hosszu, a triple gold medallist at the 2016 Rio Olympics, was strong favourite in the women's 400m individual medley - and she did not disappoint.
Dubbed the "Iron Lady", Hosszu dominated the short course world championships two years ago in Canada, carrying home seven golds.
She wasted no time getting her first victory in Hangzhou, sealing a dominant win in 4:21.40, comfortably ahead of Melanie Margalis of the United States, who won silver, more than four seconds back.
Meanwhile, Cameron van der Burgh finished third (56:90) in the men's 100m breaststroke semi-final to progress through to the final with the sixth-fastest time.
The men's 100m breaststroke final takes place on Wednesday (13:29 SA time).