Long or short, it doesn’t seem to matter to the American star.

“It’s hard the other 364 days of the year,” Ledecky said, barely breathing hard. “It’s putting the work in for practice, so when I get to this day of the meet, I can just do it. It’s routine. Just get up and know that I have the work in the bank to get up and swim those times.”

While Ledecky sucked all the suspense out of her final – she was more than 19 seconds ahead of the runner-up – King made it 2-0 over Yulia Efimova with a world-record performance in what has become one of swimming’s most compelling rivalries.

The finger-wagging American won gold at the Rio Olympics last summer after spurning Efimova and brazenly proclaiming the Russian star had no business being allowed to compete because of doping violations.

Efimova nearly broke Ruta Meilutyte’s four-year-old record in the semi-finals, giving her the prime lane in the middle of the pool.

But King, racing right beside her, was the one who came through again when it really counted. She got off to a blistering start and led all the way, touching in one minute, 4.13 seconds to shave 0.22 off the Lithuanian’s mark from the 2013 worlds in Barcelona.

King defiantly pounded the water when she saw the “WR” beside her time, then turned to congratulate the runner-up – American teammate Katie Meili, who edged out the Russian for the silver.

King was especially giddy that the US went 1-2, with Efimova relegated to a bronze.

“The rivalry is definitely there. I don’t think it’s going away anytime soon,” King said. “Obviously, it’s very awkward between the two of us. We’re competitors. We don’t really like each other too much.”

King’s bravado makes her stand out even more in the usually staid world of swimming.

“Her confidence is so impressive,” Ledecky said. “I think it’s infectious for the whole team.”

Efimova hung on the lane rope, all alone and managing a weak smile, as if she couldn’t believe the Indiana college student had beaten her again.

Still, this didn’t hurt as bad as the defeat in Rio.

“There’s still pressure from the media, but it’s more fun,” Efimova insisted. “The Olympic Games were the worst.”

Ledecky covered the metric mile in 15:31.82 – more than six seconds off her world-record pace from the world championships in Kazan, Russia, two years ago.

But she didn’t really have to push it, especially with another race to go in an event that will likely be the stiffest challenge of her bid for a record-tying six gold medals by a female swimmer at a single world championships .

She already became the most successful female ever at the worlds with her 12th career gold, breaking a tie with Missy Franklin and trailing only Michael Phelps and Ryan Lochte among all swimmers.

“I couldn’t really get my kick going into it that much,” said Ledecky, who waited 19.07 seconds for silver medallist Mireia Belmonte of Spain to finish, with the bronze going to Italy’s Simona Quadarella. “I just kind of stayed steady there.”

Ledecky had to expend more effort in the 200 free, posting a time of 1:54.69 that put her ahead of such stalwarts as Australia’s Emma McKeon, Italian world-record holder Federica Pellegrini and home-country favourite Katinka Hosszu heading into the final on Wednesday night.

Peaty, Britain’s breaststroke lion with the tattoo to match, broke a pair of 50-metre breaststroke marks – one in the morning preliminaries, another in the evening semi-finals.

Peaty’s initial time of 26.10 shaved 0.32 seconds off the standard he set at the 2015 worlds in Kazan. He went even faster a few hours later in the non-Olympic event, touching in 25.95.

When Peaty saw the time, he mouthed in disbelief, “No way.”

Masse took down another record from the rubber-suit era. 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.

“I knew I was close,” Masse said. “I was aiming to do it here.”

In all, five world records have been set in Budapest.

Masse was followed by Kathleen Baker of the USA and Australia’s Emily Seebohm.

The Chinese team had a big night, as well.

Sun Yang captured his second gold of the championships, adding the 200 free gold to his victory in the 400 free. He finished more than a half-second ahead of American Townley Haas, while the bronze went to Russia’s Aleksandr Krasnykh.

Xu Jiayu touched first in the 100 backstroke, beating Americans Matt Grevers and reigning Olympic champion Ryan Murphy, who settled for silver and bronze.