A teenage prince grabbed his first title, while a veteran queen of swimming reclaimed her throne in the first two finals of the World Championships on Monday in Budapest.

David Popovici, a 17-year-old Romanian, seized the men's 200m freestyle title in 1min 43.21sec to break the world junior record he set in qualifying.

"My goal was to go as fast as I can," he said.

Katie Ledecky, who reclaimed the 1,500m women's freestyle title, finishing almost 15 seconds ahead of her closest pursuer, said her approach was similar.

"I was having a good one, the goal was to just lock into a pace," she said.

In the women's 100m backstroke final, American Regan Smith retained her title by edging Canadian Kylie Masse.

Popovici overhauled Olympic champion Tom Dean in the third 50m-lap.

David Popovici reacts to his gold
David Popovici reacts to his gold AFP / Ferenc ISZA

"It was a strong lap, that's all I can say," said the youngster from Bucharest.

"The plan was to go out fast, not this fast though, but I guess I can surprise myself."

Another teenager, 19-year-old South Korean Hwang Sun-woo, grabbed second. Dean held on for third.

"I was on for a decent time," said Dean.

"It's not easy to see him ahead, you got to give it to him, 1:43:2, bloody hell, but that's been the case in this meet, the impossible seems impossible until it's done, and these youngsters seem to be doing just that."

Popovici became the first Romanian man to win gold at the worlds.

"The most fun is during the race because on the last lap I was saying to myself 'This is the biggest race of my life thus far and I want to make it memorable for me and everyone else'," he said.

Katie Ledecky enjoys her 1500m freestyle gold
Katie Ledecky enjoys her 1500m freestyle gold AFP / Ferenc ISZA

His time was the fourth fastest in the event's history and the second fastest, behind Michael Phelps, since streamlined suits were banned in 2009.

His swim was also faster than the best career time of another swimming legend, Australian Ian Thorpe.

"It's an honour and very flattering to be compared to Ian Thorpe," Popovici said.

"I just met him two days ago, he said if I win gold he would try to make it to award the gold medal to me, so I guess I'll see him soon. I guess Michael Phelps (is an inspiration) just like for other kids."

Ledecky touched in at 15min 30.15sec. Fellow American Katie Grimes finished 14.74sec behind with Australian Lani Pallister at 18.81.

She had won three straight 1500m world titles before illness forced her to withdraw from the 2019 final in Gwangju, South Korea. She rebounded to win gold in the Tokyo Olympics.

Ledecky, who won her first major championship gold in the London Olympics in 2012, is an old-timer by distance swimming standards but at 25 years old is 11 years younger than Chilean Kristel Koebrich who was eighth.

"I won't be doing this when I'm 36," said Ledecky.

Smith won the last world title but was third in last year's Olympics.

She made the most of the absence of Tokyo gold medallist Australian Kaylee McKeown and took revenge on the silver medallist, Canadian Masse.

Smith finished 0.18sec ahead of Masse.

American Claire Curzan continued the evening's fashion for 17-year-old medallists by finishing third.

"I knew it was going to be a really tight field and it was really going to hurt towards the end," Smith said.

"I've had a best time in three years, but tonight's about place not time."