Affronted by her rival waving her finger in a "#1" gesture after winning her semifinal, the Indiana native had chastised the Russian ahead of the race.
King told a reporter
, "You're shaking your finger No. 1, and you've been caught for drug cheating. I'm just not a fan. I'm going to go swim my heart out for USA and hopefully that turns out the best."
And on Day 3, the 19-year-old American let her swimming do the talking. After bagging the gold in one of the most anticipated swimming events of the tournament -- by just under a second -- she told NBC reporter Michelle Tafoia that she felt she had made a statement on behalf of her country and other "clean" athletes.
"I hope I (made a statement) -- that we can still compete clean and do well at the Olympic Games and that's how it should be."
King had made plain her feelings at Efimova's presence in Brazil after the Russian was cleared to compete at the 11th hour -- she didn't know she would be able to swim until Saturday -- despite having previously been found guilty of doping.
The Russian was in tears after the race.
Russian athletes booed
The decision to let Efimova and her fellow Russian swimmers compete did not sit well with spectators, who subjected the competitors to boos and jeers at the Aquatic Center -- just as the delegation had been greeted Friday
at the Games' opening ceremony.
International Olympic Committee spokesman Mark Adams said Monday it was a shame the Court of Arbitration for Sport wasn't "tougher on doping," but that Olympics officials have no choice but to accept the decision.
"These are people who have served sanctions and are now clear ..." he said. "There are athletes from many teams who have served sanctions and are able to compete."
Along with the crowd's reception for Efimova Monday, the previous day cheers in the aquatic center abruptly morphed to a bassy "boooooo" as Russians Andrey Grechin, Danila Izotov, Vladimir Morozov and Alexander Sukhorukov followed the Americans into the arena and shed their warmups.
King's best was good enough to win the race in an Olympic record time of one minute 04.93 seconds, with US teammate Katie Meili taking bronze.
"Tonight has been so crazy. My life is changing right now," King told reporters. "I'm a gold medalist and it's what I've always wanted to be -- it's an incredible feeling.
"I'm probably going to start crying. I'm usually not a crier but this is a special moment so we'll see if it gets to me or not."
Efimova, who won bronze at the London Games in 2012, was banned between October 2013 and February 2015 after testing positive for traces of the anabolic steroid DHEA.
"A week ago, I didn't even know if I could race because I'm Russian. I'm just happy to be here," the 24-year-old said, also fighting back tears.
Earlier this year, the 2015 world champion was given a provisional ban after testing positive for meldonium, but the International Swimming Federation lifted the suspension after advice from the World Anti-Doping Agency (WADA).
She was reinstated to compete at the Games last Friday after the Court for the Arbitration of Sport ruled that athletes could not be banned from competing in Rio on the basis that they have been previously sanctioned.
But Efimova was far from the only controversial athlete to attract attention on another dramatic night in the pool.
China's Sun Yang claimed gold in the men's 200m freestyle after edging out South Africa's Chad le Clos in a thrilling contest.
At the weekend, Sun was involved in a spat with Australian swimmer Mack Horton
-- who was angry that his rival was competing despite serving a doping ban in secret in 2014, news of which was only announced in China retrospectively.
For Sun it marked a personal triumph after a controversial few years outside of the pool.
In 2013 he was detained by police for seven days after being involved in an accident while driving without a license.
At last year's world championships, he was accused of assaulting a female Brazilian swimmer after an argument in the warmup pool.
Left in tears after losing out on 400m gold on Saturday, Sun roared back to win comfortably in a time of 1:44.65 to finish ahead of le Clos and US star Conor Dwyer.
American swimmer Ryan Murphy set a new Olympic record to claim gold in the men's 100m backstroke on Monday.
The 21-year-old from the University of California raced home in a time of 51.97 seconds.
Murphy's success means the US has now won six consecutive gold medals in the event.
Teammate David Plummer was also in the medals -- he took bronze, with China's Xu Jiayu claiming silver.
Hungary's Katinka Hosszu powered to her second gold medal of the Games after winning the 100m backstroke.
Hosszu, the 400m individual medley winner, pipped America's Kathleen Baker. The bronze medal was shared by Kylie Masse of Canada and China's Fu Yuanhui -- who finished in a dead heat.
The 27-year-old, who could conceivably add another three gold medals to her tally, had failed to win a medal in her three previous Olympics.
Phelps into final
Michael Phelps booked his place in the final of the men's 200m butterfly final despite finishing second in his heat.
Phelps, who won his 19th Olympic gold on Sunday
, was edged out by Hungary's Tamas Kenderesi but was still the second fastest qualifier.
Fellow American Katie Ledecky will seek to add to her gold and silver so far in the women's 200m freestyle after qualifying second behind Sweden's Sarah Sjostrom -- who has already won the 100m butterfly.