After Caeleb Dressel, one of the veterans expected to dominate the World Swimming Championships, quit the competition on Wednesday, two teenagers, 17-year-old David Popovici and Summer McIntosh, 15, leapt to the top step of the podium.
Dressel, a seven-time Olympic gold medallist, had "withdrawn for medical reasons. For his long-term health," Lindsay Mintenko, the Swimming Managing Director for the USA team, told media at the start of the evening session.
"He's just not fit to compete right now."
Popovici then claimed victory in one of Dressel's events, the men's 100m freestyle, and McIntosh won the women's 200m butterfly.
In the other individual finals, 20-year-old Frenchman Leon Marchand completed the men's medley double in the 200m and Kylie Masse won a second Canadian gold when she took the 50m women's backstroke.
The evening ended with another of the US team's veteran stars, Katie Ledecky, carving a place in the record books with the winning women's 200m relay team.
It was Ledecky's 21st world championship medal, more than any other woman.
Australia took silver while Canada, with McIntosh collecting a second medal of the day, held on for bronze.
Dressel had already won two golds in Budapest before his last race on Tuesday morning. It was a heat of the 100m freestyle and the Olympic champion qualified only second fastest behind Popovici.
The teenager was asked after the final if he had scared off Dressel.
"I don't think so, I think he is too big of a boy to be running away from someone like myself or frankly anyone but I hope he's OK and I hope he'll come back stronger."
Popovici, who had become the first Romanian man to win a world title when he took the 200m free, became the first man to do the 100-200 freestyle double at the world championships since American Jim Montgomery in 1973.
Dutchman Pieter van den Hoogenband repeated the rare feat at the Sydney Olympics in 2000.
"I'm glad we got to write a small page in swimming history," said the smiling Popovic. "Some would say a huge page, but we like to keep it low key."
"I'm glad to have got two golds now, I think it will be pretty heavy carrying them."
Popovici edged Frenchman Maxime Grousset by 0.6sec and Canadian Joshua Liendo by 0.13.
- 'Smart race' -
"In the 200 I was surprised by how much I won by. This time I was surprised by how little we won by," he said, adding he preferred the 200m event.
"I consider 200m more of a smart race," he said.
In the two-lap 100, he said, "we have to go out as fast as we can and come back as fast as possible. It's an animal instinct race."
Popovic prepared for a career as a sports star by attending a bilingual school in Romania and speaks fluent English.
"I haven't finished high school yet I don't even drive yet," he pointed out.
At 15, McIntosh is even younger.
The Canadian broke the world junior record as she added the 200m butterfly to the silver she had won in the 400m freestyle on the opening day.
McIntosh beat Hali Flickinger of the United States by 0.88sec with Zhang Yufei of China third.
"I just literally gave it my all and did everything I could, and put in all my energy and all my focus, and just stretched for the wall and put my hand on the wall as fast as I possibly could," McIntosh said.
Marchand, who claimed the 400m title on Saturday, grabbed the lead in the breaststroke and held it on the last lap to finish in 1min 55.22sec, edging out American Carson Foster and Japanese bronze medallist Daiya Seto.
The second Canadian gold medallist, Masse, a relative old timer at 26, had won gold in the 100m backstroke at the previous two world championships and a silver in Budapest.
She has never won a major medal at the shortest distance.