Lilly King, right, celebrates her gold medal in the 100-meter breaststroke with Katie Meili, who took bronze. (Jonathan Newton/The Washington Post)

It would be a stretch, by almost any definition, to refer to Lilly King and Yulia Efimova as swimming rivals. The world-class breaststrokers, the former an American, the latter a Russian, had never raced each other in the same heat as best as anyone could determine, and as recently as Friday night they were not even entered together in the 100-meter breaststroke at the 2016 Rio de Janeiro Olympics. Only King was.

But then FINA, swimming’s world governing body, decided without explanation the night before the competition to allow Efimova into the Olympic meet, despite ties to the current Russian doping scandal, and then King expressed, through words and gestures, her displeasure with that decision. And then, at 10:54 p.m. local time Monday in Rio, they found themselves crouched on the starting blocks next to each other for a race that emerged out of nowhere to become one of the most highly anticipated duels of these Olympics.

And it was a duel that delivered, both in the pool — where King held off Efimova down the stretch to touch in 1:04.93 and win the gold medal, punctuating the win by drifting toward Efimova (1:05.50) and slapping the water in the silver medalist’s lane with her right hand — and out.

“It’s incredible — winning the gold medal and knowing I did it clean,” said King, 19. Asked about the post-race splash in Efimova’s lane, she said, “I think I just floated over that way. I wasn’t actually planning to hit the water in her lane. I just kind of hit the water.” King pointedly swam to hug teammate Katie Meili, the bronze medalist at 1:05.69, but ignored Efimova.

“I don’t think she really wanted to be congratulated by me at that point,” King said, “so I figured I should stay out of it.”

Efimova was in tears after the race, taking a few minutes to compose herself before saying, “A week ago, I didn’t even know if I could race, because I’m Russian. I’m just happy to be here.”

At an awkward and emotional news conference less than an hour later, Efimova and King sat at the same table, with Meili in between, fielding questions about each other and about the issue that divides them, never so much as looking at each other.

“After standing up for what I believe is right, I felt I had to perform even better,” King said. “This is a victory for clean sport.”

Efimova, 24, served a 16-month suspension in 2014 and 2015 following a positive test for DHEA, then also reportedly tested positive for meldonium this March, triggering another suspension. But the second suspension was provisionally lifted when the World Anti-Doping Agency relaxed its rules regarding meldonium, and the Court of Arbitration for Sports ruled the International Olympic Committee’s blanket ban on Russian athletes who have served prior doping suspensions was “unenforceable” — opening the door for Efimova’s participation in Rio de Janeiro.

“For me, it’s very hard swimming today,” Efimova said during the news conference, sounding as if she were near tears. “. . . I can’t understand what’s going on. Usually in Olympic Games, all wars stop, [but] this is not fair.”

King’s victory highlighted a stellar night for the Americans, who won six of the available 12 medals across four finals. Two of them were gold — King’s and Ryan Murphy’s in the 100 back, in which Murphy set an Olympic record of 51.97 seconds, edging China’s Jiayu Xu (52.31). American David Plummer, a first-time Olympian at age 30, took the bronze medal (52.40).

University of Virginia physics professor Lou Bloomfield explains some of the fundamental forces at work in Olympic freestyle swimming, and how swimmers can use science to get ahead. (Thomas Johnson,Julio Negron,Danielle Kunitz,Osman Malik,Dani Johnson/The Washington Post)

If King vs. Efimova was the biggest head-to-head confrontation Monday night, there was also plenty of groundwork laid for juicy showdowns still to come.

The semifinals of the women’s 200 free featured a particularly compelling matchup between Katie Ledecky, the two-time Olympic gold medalist from Bethesda, and Swedish standout Sarah Sjostrom. Sjostrom and Ledecky, ranked first and second in the world this year in the 200, were both coming off world record swims the night before — Sjostrom in the 100-meter butterfly, Ledecky in the 400-meter freestyle.

Racing in adjoining lanes in the same semifinal heat Monday, Sjostrom out-touched Ledecky by sixteen-hundredths of a second, 1:54.65 to 1:54.81, to set up a rematch in Tuesday night’s final, in which they will be the top two seeds.

“It was a tough race. I knew my heat was going to be pretty fast,” Ledecky said. With no swims in the afternoon preliminary heats Tuesday, Ledecky said she planned to sleep in. “I get to regroup and get ready. . . . It’s the third round that counts.”

American Missy Franklin, a four-time gold medalist in 2012 who has struggled to regain her London form, finished last in the first semifinal heat and failed to qualify for the final. She has two more events remaining — the 200 back and the 4x200 free relay — in which to get to the medal stand.

“You just put it behind you,” Franklin, 21, said. “I’ll talk to my coaches and see what happened. . . . It’s just one of those things I’ve been trying to wrap my mind around this year — what’s been going wrong. But I haven’t been able to figure it out.”

In the semifinals of the men’s 200 fly, American Michael Phelps, the most decorated Olympian of all time, took on rival Chad le Clos, with both moving on to the final in easy fashion, Phelps in 1:54.12, good for second place behind Hungary’s Tamas Kenderesi (1:53.96), and le Clos in fourth place at 1:55.19. Before the race, television cameras caught le Clos shadow-boxing and dancing directly in front of a seated Phelps, who stared straight ahead with a scowl on his face.

“Everyone has their own race strategy,” Phelps said. “If that’s his, that’s his.”

The King-Efimova affair began Sunday night during the semifinal of the women’s 100 breaststroke, when Efimova won easily in the earlier heat and waved a single finger — signaling “number one” — at the television camera. In the ready room preparing to walk onto the pool deck for her heat, King, who saw Efimova’s gesture on a television, wagged her finger back and forth in disapproval, her gesture also caught by television cameras.

Ray Looze, King’s coach, said King never realized the gesture would be televised.

“They had a team meeting today, and she goes, ‘Hey, girls, I just want y’all to know they film you in the ready room,’ ” he said. “So it’s not something that she wanted to make public.”

Looze said he advised King not to speak out about doping during the Olympics.

“I said, ‘Take the high road. Don’t say anything. Don’t get involved. Let’s let our actions speak louder than our words,’ ” he said. “But they’re adults and they have opinions. And . . . if [the authorities] aren’t going to do anything about [doping], the athletes are going to rise up.”

Asked specifically about U.S. sprinter Justin Gatlin, who has served two doping suspensions, King said, “Do I think people caught for doping offenses should be on the team? No.”