France's Leon Marchand smashed Michael Phelps's last remaining world record on Sunday while Australia's Ariarne Titmus blew her 400m freestyle rivals out of the water at swimming's world championships.
Marchand clocked 4min, 02.50sec to win the 400m individual medley, taking more than a second off Phelps's record.
The 21-year-old is set to be one of the faces of next year's Paris Olympics and he warned that "the best is yet to come".
"It was insane," Marchand said.
"That was one of the most painful things I did. That was amazing to do it here. The time is crazy."
Phelps's world record was one of three to fall on a dramatic opening night in the southern Japanese city of Fukuoka.
Titmus reclaimed her world record in the women's 400m freestyle, while Australia's women's 4x100m freestyle team also improved the mark they set two years ago.
Phelps's benchmark of 4:03.84 had stood since the 2008 Beijing Olympics and was the longest-held world record in swimming.
Carson Palmer, who finished second behind Marchand in a time of 4:06.56, said it was "pretty cool" to watch the Frenchman break it.
"Obviously the goal is always to win and Leon is a great competitor," said Foster.
"Being the closest person to history was pretty cool, getting a front-row seat to that."
Japan's Daiya Seto finished third in a time of 4:09.41.
- 'Really excited' -
Titmus earlier dominated in one of the most hotly anticipated races of the championships, the women's 400m freestyle.
It pitted Olympic champion Titmus against world record-holder Summer McIntosh and defending world champion Katie Ledecky.
Titmus made short work of her rivals, coming home in 3 min, 55.38 sec to win the title and take back the world record that McIntosh had wrested from her in March.
Ledecky finished the race second in 3:58.73 but McIntosh could only place fourth, with New Zealand's Erika Fairweather taking third in 3:59.59.
Titmus said she "honestly did not think about getting the world record back".
"I just wanted to come here and swim the way that I felt I was capable of swimming," she said.
"It was obviously enough to get the record back."
It was also the first time that Titmus, Ledecky and McIntosh had gone head-to-head since Titmus won at the Tokyo Olympics two years ago.
"I was really excited for this race coming in tonight," Titmus said.
"Me and Katie and Summer always put up a great fight and I'm glad that we could put on a show."
On a successful night for Australia, Sam Short won the men's 400m freestyle, pipping Tunisia's Olympic champion Ahmed Hafnaoui.
The 19-year-old was on pace to break the world record at the 300m mark but came home in 3 minutes, 40.68 seconds, missing the record by 0.61 sec.
"With 100 to go, I was just going for the win because Ahmed is the Olympic champion so this was going to go down to the wire," Short said.
"I just put my head down at the end and I don't know how I beat his long arms to the wall but I did it."
Hafnaoui finished second on 3:40.70, while Germany's Lukas Martens was third on 3:42.20.
Defending champion Elijah Winnington of Australia finished seventh.
In the women's 4x100m freestyle relay, the Australians came home in a world record time of 3min, 27.96sec to take gold ahead of the United States on 3:31.93 and China on 3:32.40.
The Australian team of Mollie O'Callaghan, Shayna Jack, Meg Harris and Emma McKeon beat the record of 3:29.69, also set by Australia at the Tokyo Olympics two years ago.
"It's really exciting to start the night off with a world record in the relay," said O'Callaghan.
"I'm very pleased and I’m proud of these girls."
Australia's men's 4x100m freestyle team also claimed gold, with Italy taking silver and the United States bronze.
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