Two days after team mate Kim Woo-min took a surprise gold in the men's 400m freestyle, Hwang stormed to victory in one minute and 44.75 seconds at the Aspire Dome pool in Doha to improve upon the bronze he won at Fukuoka last July.
"Clearly, we have great relationships within the team," Kim Woo-min told reporters.
"We're all very close and train together. I think that brings synergy."
Having charged out early, Hwang had to find another gear in a thrilling final lap to see off Lithuanian runner-up Danas Rapsys by 0.30 seconds and American bronze-winner Luke Hobson.
Rapsys celebrated a long-awaited world medal, nearly five years after being stripped of the 200m gold at the 2019 event in Gwangju for a false start.
"I was an unofficial gold medallist in 2019, now it’s silver, but in my mind, it’s like gold, and it’s official this time," Rapsys said with a sigh.
Having bombed out of the 400m freestyle preliminaries as favourite on Sunday, Tunisian Hafnaoui exited the 800, qualifying 18th fastest.
His time of 7:51.72 was more than 14 seconds off his championship-winning swim at Fukuoka last July.
The Tunisian had been less than bullish about his form on the eve of the meeting but the Doha results raise more questions about his preparations for the Paris Olympics.
ITALIAN JOB DONE
Italian distance swimmer Simona Quadarella has no such concerns and confirmed she will be the chief threat to American great Katie Ledecky in the 1,500m freestyle at Paris.
With Ledecky absent, Quadarella romped to her second world title, and first since 2019, in 15:46.99, nearly 10 seconds ahead of China's Li Bingjie, also the 400m silver medallist.
Quadarella was happy with her swim but gave herself no chance of beating Ledecky at the Paris Olympics.
"No, I don't think so," she told Reuters with a laugh.
In a depleted women's 100 metres backstroke field, Claire Curzan stormed to victory for the United States in a personal best 58.29, adding to her 100m butterfly silver.
A late illness before the U.S. trials for Fukuoka cost Curzan a place on the team but the versatile Virginia University swimmer has made the most of Doha.
Her U.S. team mate Hunter Armstrong took the men's 100m backstroke gold in 52.68 seconds, pipping Spain's Hugo Gonzalez by 0.02 seconds.
"To get to share the podium with Hugo - this dude is my brother and a training partner, so it is awesome," said Armstrong, who took the bronze in Fukuoka.
"It is just one thing to win the title but to share it with the person that you love, that makes it much better."
China's Tang Qianting capitalised on the absence of U.S. world record holder Lilly King and a slew of other big names to win the 100 breaststroke title in 1:05.27, with Dutchwoman Tes Schouten runner-up and Hong Kong's Siobhan Haughey third.
"I entered this for fun," said freestyle specialist Haughey.
Haughey earlier qualified second for Wednesday's 200m freestyle final.
World record holder Adam Peaty, who took bronze in the 100m breaststroke on Monday, eased into the final of the non-Olympic 50m event, with Australia's Sam Williamson topping qualifying.
Rising Irishman Daniel Wiffen qualified second for the 800m freestyle final on Wednesday and is riding high after breaking the short course world record in December.
Japan's Tomoru Honda, the 200m butterfly silver medallist at the Tokyo Olympics, will bid for his first world title in the event on Wednesday.
