Hassiem, 30, recounted his bronze-medal-winning effort Monday with the Associated Press, where he said he used the great white shark attack that left him without his right leg as motivation when he swims.
"My little secret is obviously that I just try and imagine I'm in the ocean and I've got a 4 1/2-meter great white shark at my feet," Hassiem told the AP. "It's definitely good motivation to swim fast."
Hassiem was bit by the great white shark six years ago off the coast of Cape Town while trying to save his brother Tariq from the vicious predator. He distracted the shark by slapping the water.
The maneuver saved Tariq, but the great white shark attacked Hassiem, biting his right leg and dragging him, according to the AP.
"I gave one last enormous push and heard a great snapping sound," Hassiem told the AP.
The South African is not bitter about losing his leg. If anything, the Paralympic swimmer is grateful that his actions saved his brother's life.
"I believe I lost my leg for a good reason," he said. "Losing a leg is nothing compared to losing my brother, so I'm just trying to make the best of it."
The attack even led to a nickname -- "Sharkboy" -- according to his official London 2012 Paralympics bio.
Hassiem was encouraged to compete in the Paralympics by fellow South African Paralympian Natalie Du Toit, who called him "an inspiration" in an interview with the AP.
"It's been great to see Achmat do better and better in the last couple of years," she said. "I think he's become a role model because people look at him and wonder how they would cope in that situation. He's just gotten on with it and kept going."
Hassiem's run at the 2012 Paralympics in London is just beginning. He is scheduled to compete in the 400-meter freestyle and 100-meter freestyle. Both races will be conducted Wednesday.