Although the athlete represents the peak of human athletic prowess, he still can only swim at a top speed of 5-6mph (8-10km/h) an hour, slower than the shark.
To even things up, the US swimmer wore a "monofin" for the open water race in South Africa, increasing his speed significantly but still not taking it anywhere close to the at least 25mph a Great White is capable of in short bursts.
And - spoiler alert - the athlete has revealed they were not in the water "at the exact same time" for safety reasons.
But humans have long pitted themselves against dangerous animals, often ones they know are much faster.
They have done this for money, to draw attention to a cause, to create a spectacle, and perhaps also out of an inflated sense of what humans are capable of.
Here are four other times man has raced beast.
South African rugby star v a cheetah
Bryan Habana, one of the fastest players in international rugby, decided to take on the world's fastest land animal in 2007 as part of an event sponsored by a conservation group.
Habana is quick, but not Usain Bolt quick - running the 100m in about 11 seconds (compared with Bolt's 9.58 world record).
Still, the then 23-year-old, keen to raise awareness about the decline of the cheetah, fancied himself in with a chance.
But tempting the cheetah was a dangling leg of lamb, which it chased during the race as Habana, who was given a significant headstart, gave it all he had.
The end was close - but the cheetah just got over the line first. Habana asked for a re-run and was soundly beaten.
Italian swimmer v dolphins
Filippo Magnini, a former world champ in the 100m freestyle, took on two dolphins in a pool near Rome in 2011.
Given the animals' clear advantage, the Italian only had to swim one length of the pool, while they had to swim two.
But that didn't make a difference and the man nicknamed 'Superpippo' was pipped at the post.
He said later that he fell "a bit in love" with Leah, one of the dolphins.
Jesse Owens v horses
The black US track and field athlete won a string of victories at the 1936 Berlin Olympics in front of Adolf Hitler, who had been hoping for a games that would demonstrate the Aryan superiority he believed existed.
Owens later struggled financially back home in a country where racism remained rife and his sport was not professional.
To make money, he competed against racehorses in front of dazzled crowds.
He is said to have had the starting gun go off close to the horse, stunning it and allowing him to get away ahead (he also had a head start). Though this strategy worked most of the time, he didn't always win.
Later, more opportunities became available to Jesse Owens and, among other roles, he served as Ambassador of Sports under President Eisenhower.
American football player v ostrich
NFL wide receiver Dennis Northcutt easily beat an ostrich named Thelma in 2009 for a TV show called Sport Science.
But in that initial race, a fence separated the pair, and it was obvious the animal wasn't giving it her best.
In a second race, this time inside the ostrich's enclosure, Dennis was soundly beaten, as the ostrich leapt away and he was left chasing it through the dust.