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Michael Phelps retired from competitive swimming following the 2016 Summer Games in Rio as the most decorated Olympian of all time -- with 28 medals, including 23 gold. But Phelps’ most recent race – which will air Sunday on the Discovery channel – featured his most challenging opponent -- a great white shark.

“It was a tough a race, probably the hardest race I’ve ever had,” Phelps told Fox News, declining to disclose the outcome.

Though Phelps has lost very few races in his career, not many people gave him a chance against the shark -- perhaps because great whites can reach speeds up to 25 mph, while Phelps clocks in around 6 mph, per ESPN.

Not even Phelps’ Olympic teammate, Ryan Lochte, had faith that Phelps could outswim the shark.

“Hell no,” Lochte told TMZ.

Swimming in open water, “with current and cold water,” was also a big adjustment from the typical pool setting, Phelps said. However, Discovery outfitted the Olympian with some technology to help even the odds.

People-Michael Phelps

FILE - In this Wednesday, Aug. 10, 2016, file photo, United States' Michael Phelps, left, and Ryan Lochte celebrate winning the gold medal in the men's 4x200-meter freestyle relay during the swimming competitions at the 2016 Summer Olympics in Rio de Janeiro, Brazil. Phelps isn't sure what kind of moves Lochte has planned for "Dancing with the Stars." "I've never seen him dance," the 23-time Olympic gold medalist said Friday, Sept. 2, at the "Call of Duty" fan convention in Inglewood, Calif. (AP Photo/Lee Jin-man, File) (AP)

“We were able to add some help with the monofin, and being able to have a wetsuit gave a little bit of help towards me,” Phelps said. “I think the odds are pretty stacked in the shark’s favor, but you guys will have to wait and see, to see what happens.”

The 100-meter race took place off Cape Town, South Africa, and had been on 32-year-old Phelps’ bucket list “forever,” he said.

“It was honestly just something I’ve always wanted to do, being in the water with a shark,” Phelps said. “Whether it’s in a cage, or just swimming freely in the ocean. … Having the chance to literally see these animals up close and personal in their home, it topped so many things that I’ve done -- just being able to see how they truly are.

“I was literally within a foot from two different sharks that were that close to my face -- one in a cage, one out of the cage.”

Phelps wasn’t the first Olympian to test his skill against an animal. Track and field legend Jesse Owens once outran a horse in Havana – after getting a 40-yard head start.

So, who knows? Maybe Phelps has a shot after all.

“Phelps vs. Shark: Great Gold vs. Great White” will air at 8 p.m. ET Sunday on Discovery, as part of the channel’s annual “Shark Week.”