The best female 200-meter freestylers in the world had their moment, their chance to load up on gold medals and sleep easy afterward. For the past three years, they could watch the Katie Ledecky phenomenon take over their sport from a relatively safe distance, comfortable in the knowledge that for all her historic, world record prowess in the freestyle distance events — the 400, 800 and 1,500 — she wasn’t yet the same overpowering force in the 200. She hadn’t yet come for their kingdoms.

That period of blissful comfort ended Wednesday night. It ended on Day 4 of the FINA World Swimming Championships at Kazan Arena, where Ledecky, the 18-year-old champion from Bethesda, ran down a decorated field over the final 50 meters in one of the most anticipated event finals at these world championships to lock down her third gold medal of the meet — the only three golds for the United States.

The next time she swims an individual event here, the 800 meters, with preliminary heats Friday and finals Saturday, she will be shooting to become the first swimmer in history, male or female, to sweep the 200, 400, 800 and 1,500 freestyles at the same world championships — something that may one day become known as the Ledecky Slam.

“I really didn’t know that nobody has ever [won all four] until a month ago. I wanted to do well in all of them before,” Ledecky said. “But that gives me a little more motivation, and yeah, that would be really special.”

Katie Ledecky broke her own world record in the womens' 1500-meter freestyle at the world swimming championships in Kazan, Russia. Thirty minutes later, she was back in the pool again to qualify for the 200 meter freestyle final. (Jayne W. Orenstein/The Washington Post)

Exactly one year before the Opening Ceremonies of the 2016 Rio de Janeiro Olympics, Ledecky managed to do something Wednesday that felt even more significant than the golds she took in the 400 free Sunday or the 1,500 free Tuesday. She entered this week as the world record holder in both of those, and when she blew away those fields — coming just shy of a world record in the 400 and destroying her mark (for the second time in two days) in the 1,500 — it was completely expected.

Her win Wednesday night in the 200 — an event she chose to skip at this same meet two years ago, then won a year ago at the Pan Pacific Championships against a lesser field — was not expected or, at the very least, far less certain.

But now that Ledecky has won the 200 — with a time of 1 minute 55.16 seconds, edging out runner-up Federica Pellegrini of Italy and defending champion Missy Franklin of the United States — it felt as if something had just changed in the sport, as if the 200 was now tilting toward Ledecky, dangerously close to joining the other, bigger numbers already residing under her ledger, as belonging to her.

“I don’t know if she has any weaknesses,” Franklin said. “If she does, we haven’t seen them yet. . . . It’s just awesome to sit back and watch her go.”

There is now a very good chance Ledecky heads back to Bethesda on Monday with five gold medals in her carry-on — to go with the four she won in 2013, plus the 800 gold from the 2012 London Olympics. Next up, on Thursday, is the 4x200 freestyle relay, which she will likely anchor, followed by the two-day, two-tier 800 free, for which she is also the world record holder, with no one within 10 seconds of her this year. Franklin, with six golds in 2013, is the only woman to have won more than five in a single world championships.

Ledecky entered Wednesday having never lost a race in a major international meet. But if there was reason to think this one could trip her up, it was simply because of her relative inexperience or her fourth-ranked status in the world entering the day or the presence in the field of two of the three ranked ahead of her (Pellegrini and Sweden’s Femke Heemskerk) and the two ranked just behind her — a group of swimmers who collectively owned six of the fastest 10 times in history in the event.

“That’s a tough field, with an extremely deep, talented, fast group of individuals,” said Bruce Gemmell, Ledecky’s coach. “That’s a tough race to win.”

Olympic gold medalist Katie Ledecky broke multiple world swimming records before graduating high school. Now in her senior year in Bethesda, Md., she’s wrapping up her high school swimming career and looking forward to World Championships this summer. (Zoeann Murphy/The Washington Post)

Perhaps the best reason to bet against Ledecky, if you were inclined toward sucker bets, was because she was coming off a grueling “double” the day before, when she raced (and won, with a world record) the 1,500, followed by the 200 semifinals, with only half an hour in between.

Ledecky ended Tuesday night with a massage, slept well and came to the pool in the morning for an easy warmup swim. Gemmell met her there and avoided asking about her condition because he was scared of what he might hear.

“The last thing I wanted to hear her say,” he said, “was, ‘I feel like crap.’ ”

She had drawn Lane 7 for these finals rather than her usual Lane 4, where top seeds are placed, and it turned into a slight advantage, she would say later, because it allowed her to keep tabs on all but one fellow competitor, Australia’s Emma McKeon in Lane 8, when she turned her head to breathe.

She hit the first turn in third place and was in fourth at the halfway point. But her third lap (29.56) was the fastest in the field, moving her up to second, and she turned for home in a lead pack that also included Heemskerk, ranked No. 1 in the world this year, and Franklin. But Heemskerk inexplicably faded, dropping a full second in the final lap, while Ledecky and Pellegrini, in fourth entering the final 50, surged.

Ledecky, with one last revolution of that familiar, long stroke, touched the wall first, sixteen-hundredths of a second ahead of Pellegrini. If there is one thing Ledecky has proved the past two nights, with her furious final laps in the 200 semis and finals, it is that she may be the best closer in the sport.

Over the past few years, the 200 free has gone from an event Ledecky toyed with as a potential avenue for a relay medal to one she began to take seriously (making the semis at the U.S. Olympic trials in 2012) to one she knew she could win (the 2014 Pan Pacs victory), and now, to one she can see herself winning in Rio, on the sport’s biggest stage, against a field that will be even stronger.

“I’ve just been chipping away [at the race], and it’s pretty special to be able to swim the 200 here,” she said.

Gemmell wouldn’t bite on a question about the significance of Wednesday night’s win for Ledecky’s future, in Rio or beyond, but with the women’s 1,500 absent from the women’s schedule, the only direction for Ledecky to go to expand her options was toward the shorter races. And only someone who hasn’t been watching would now call her anything less than the favorite in the 200 in Rio.

“I think the significance is she is a medal contender in the 200 free in Rio,” Gemmell said. “But if she hadn’t won that race, I think I would still say she’s a medal contender in the 200 free in Rio. I don’t know this changed that outlook any.”

But for the rest of the field of world-class swimmers, the outlook for Rio and beyond has changed significantly — and not in a way any of them are going to be happy about.