The magazine dubbed Phelps the "greatest Olympian of all time" after he became the most decorated athlete to compete in the Olympics, taking home his 28th overall medal at Rio over the summer.
Unlike most athletes, Phelps was able to call it quits while still on top, and he opened up to SI about what he'll miss most about competing in the Olympics.
"The hardest thing is going to be not having the chance to represent my country," Phelps said. "Not having the chance to stand on the medal platform and hear the national anthem. I'm at peace with how things ended. I'd rather have a healthy body in 20 years than kill myself more now. To me, it's a no-brainer."
Phelps' 2016 retirement announcement marked the second time he has walked away from the sport. The swimmer first retired in 2012, only to propose a comeback in April 2014. This time, Phelps seems resolute about staying out of the pool.
"I could probably go four more years. But could I put in the true, honest hard work? Probably. But I have so many other things going," Phelps explained. "And there's no reason. I'm tapping out. I'm closing. I'm done."
Still, friend NBA star Chris Paul predicts there is one factor that could give him the fire to compete again.
"I played golf with Chris Paul in September," Phelps recalled. "He didn't go to Rio, and he told me, 'Wait until your son is old enough and he says, 'Daddy, why aren't you out there?'"