They never said he'd go head-to-head with the real thing, but when viewers tuned in to watch Michael Phelps race a great white shark, many were disappointed.

That's because the 23-time Olympic gold medalist didn't race a shark at all, but a computer-generated simulation.

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And he didn't even win.

Turns out “Michael Phelps races a shark” was really just “Michael Phelps swims alone and then compares his time to a shark’s time.”

— Gary Parrish (@GaryParrishCBS) July 24, 2017

Phelps lost the race — part of the Discovery Channel's Shark Week special Phelps vs. Shark: Great Gold vs. Great White — to the simulated shark by two seconds.

Folks who tuned in to the special saw the Great White in one lane and Phelps right next to it while wearing a special fin to mimic a shark's tail.

But in reality, they were never in the water together. Instead, it was a CGI shark animated into footage of Phelps swimming in the waters off South Africa.

I know it's stupid to think michael phelps was actually going to be racing against a real shark but I'm stupid and upset

— Nicole (@nicoleee_adams) July 24, 2017

Despite clues Phelps gave in interviews leading up to the "race," many viewers didn't seem to understand that the Olympian would never actually come fin-to-fin with the man-eater.

"Turns out “Michael Phelps races a shark” was really just 'Michael Phelps swims alone and then compares his time to a shark’s time,'" Gary Parris, a sports analyst for CBS Radio tweeted, part of a chorus of posts about the so-called "race."

Another Twitter user wrote: "i spent 60 minutes of my life watching a build up to Michael Phelps race a fake computer generated shark..."

While admitting it was probably a long shot, another viewer expressed her extreme disappointment.

"I know it's stupid to think Michael Phelps was actually going to be racing against a real shark but I'm stupid and upset," tweeted @nicoleee_adams.

Rematch? Next time..warmer water. #SW30 @Discovery @SharkWeek

— Michael Phelps (@MichaelPhelps) July 24, 2017

When he took to Twitter himself after the show aired, Phelps declined to address any of the unmet expectations.

Watch: College Student Survives a Charging Shark by Stabbing It With Spear Gun

"Rematch? Next time..warmer water," Phelps wrote.

Judging from the backlash, that rematch seems unlikely. 

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