American great Ledecky cruised to a dominant 1,500m freestyle gold, touching the wall in 15:26.44s, having threatened her own world record of 15:20.48s.
The masterful win in Singapore moved Ledecky four short of the retired Phelps’s career tally of 26 world gold medals.
“Growing up it was very inspiring to watch World Championships, Olympic Games that Michael was in, trying to learn from the very best,” said Ledecky, 28.
“I was always a student of the sport.”
Ledecky’s win gave the American the sixth world 1,500m freestyle title of her career.
She is also a nine-time Olympic gold medallist.
Second was Simona Quadarella of Italy in 15:31.79s and third Australia’s Lani Pallister in 15:41.18s.
Ledecky was the red-hot favourite going into the race, having qualified for the final over 10 seconds faster than nearest challenger Pallister.
She also went into the race owning the top 24 1,500m freestyle times ever, and set the second-fastest time in history in April behind her own record.
Pallister pushed her with a fast early tempo and Ledecky was on world record pace until late in the race.
In the end she fell short of the mark but still clocked the fifth-fastest time ever.
“Lani took it out there. I knew she would be out fast and I just wanted to get out fast, but comfortable enough that I could go from there,” said Ledecky.
“I’m happy with the time and happy with the swim.”
It is Ledecky’s first gold medal of the Championships, after picking up a bronze behind Summer McIntosh in the 400m freestyle on Sunday’s opening night in Singapore.
Ledecky is set to go head-to-head with Canadian phenomenon McIntosh again in the 800m freestyle.
Popovici claimed a thrilling 200m freestyle gold – then revealed he had very nearly pulled out.
The 20-year-old Romanian reeled in US rival Luke Hobson to take the title in 1:43.53s, with Hobson settling for silver (1:43.84s) and Japan’s 18-year-old Tatsuya Murasa taking bronze (1:44.54s).
Popovici said afterwards that he had seriously considered withdrawing from the competition before it started, even going as far as looking at flights home.
“It was a mental thing and it had to do with the fear of seeing my real potential,” said Popovici, who reclaimed his world crown from 2022.
“That’s very scary. And so I’m really glad I didn’t quit.”
‘Nerve-wracking’
Australia’s McKeown beat a recently dislocated shoulder, nerves and old rival Regan Smith of the US to win the 100m backstroke gold, narrowly missing the world record.
McKeown powered to the wall in a championship record 57.16s, with Smith having to once more settle for second, just 0.19s behind.
Katharine Berkoff, also of the US, took bronze.
McKeown’s victory underlines her status as the undisputed force in women’s backstroke.
“It’s always very nerve-wracking, especially off the back of the Olympics,” said the 24-year-old, who won the 100-200m backstroke double at the Tokyo and Paris Olympics.
“I was probably a bit more nervous than I would like to have been tonight.”
McKeown’s latest triumph came after hurting her shoulder in the lead-up to Singapore.
“It’s been quite irritated but I’ve got a good medical team and physios to help me get through.”
Germany’s Anna Elendt won the first world title of her career in the women’s 100m breaststroke, coming home in 1:05.19s.
American Kate Douglass was second in 1:05.27s, with China’s Tang Qianting third in 1:05.64s.
South Africa’s Pieter Coetze won the men’s 100m backstroke in 51.85s in a surprise, ahead of Italy’s Thomas Ceccon (51.90s) and France’s Yohann Ndoye-Brouard.