This timing is 0.44sec off his Olympic record of 50.39sec, set in Rio de Janeiro, Brazil previous year, as American Caeleb Dressel won in phenomenal fashion, clocking 49.86sec to come within 0.04sec of compatriot Michael Phelps' world record of 49.82sec set with a supersuit in the 2009 edition.
The 20-year-old now has six gold medals at the meeting and can tie Michael Phelps' record of seven - set at the 2007 championships in Melbourne - with success in the men's 4x100m medley relay on Sunday. "You have to enjoy the moment and refocus very quickly".
"I don't consider myself a talented guy, but I'm sure a hard worker", said Fratus.
The 20-year-old American then had 33 minutes to ready himself for the men's 100m butterfly.
He never trailed and delivered two lengths of sizzling pace in 49.86 seconds.
The former Clay High School swimmer captured his seventh gold medal of the FINA world championships, tying the all-time record of Michael Phelps, in the meet's closing race Sunday in Budapest, Hungary.
"I think I only had to run twice", Dressel said with a smile.
Dressel's speed, coupled with the sheer noise inside the arena, clearly lifted Kristof Milak of Hungary who posted a world junior record time for silver.
A time of three minutes and 27.91 seconds saw the US beat Great Britain by just over a second, with Russian Federation taking bronze.
He already has big plans for Australia's 4x200m freestyle relay team at the 2018 Gold Coast Commonwealth Games.
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The males are 18-year-old Richard Irakoze and Aristide Irambona; 17-year-old Kevin Sabumukiza; and 16-year-old Don Ingabire . As NPR's Laurel Wamsley reported , President Trump "intervened to find a way to permit the girls entry".
The 20-year-old Ledecky won the 800-meter freestyle in 8 minutes, 12.68 seconds, which was almost 8 seconds off her world record at last summer's Rio Olympics, where she won five gold medals.
Ranomi Kromowidjojo of the Netherlands took silver.
Ledecky won in 8:12.68, which was almost 8 seconds off her world record at Rio de Janeiro.
Sarah Sjostrom of Sweden set a new world-record for the women's 50m freestyle by clocking 23.67 secs in Saturday's semi-finals.
United States swimmer Caeleb Dressel finished overall first in the heats after touching the wall in 50.08s. Nobody has ever done it at a World Championships.
Hopes had rested heavily on Seebohm with Australia on the cusp of finishing without a world title for the first time in 31 years.
She won the women's 100 backstroke in 51.10 - 0.02 better than the mark set by Britain's Gemma Spofforth's at the 2009 worlds in Rome, the last hurrah for the performance-enhancing attire that allowed swimmers to essentially rewrite the record book.
Masse was followed by Kathleen Baker of the USA and Australia's Emily Seebohm.
To end the night, the USA freestyle team's victory never looked in doubt.