But no matter the outcome, saying goodbye to the pool won't be easy for the retiring Olympic athlete – nor his family.
"It'll be a big moment," his mom Debbie, 61, a middle school principal outside Baltimore, Md., recently told PEOPLE. "I'll pull out my box of [tissues], because it's going to be emotional."
Recently Phelps confirmed that this would be his fourth and final Olympic Games.
After qualifying for eight events in the U.S. Olympic swimming trials in Omaha, he's opted to swim seven: the 200 and 400 individual medley, the 100 and 200 butterfly, and all three relays.
"It's so much smarter for me to do that," Phelps told the Associated Press July 2 after scratching on the 200-meter freestyle. "We're not trying to recreate what happened in Beijing. It just makes more sense."
What happened in Beijing, of course, was magic – Phelps made history by winning eight medals, the most of any athlete in a single Olympic Games, besting American swimmer Mark Spitz's record seven in 1972.
Phelps at 27So what can fans of the most exciting swimmer in U.S. Olympic history expect this time around?
"Maturity," says Debbie of her son, who turned 27 during trials. "He's an old man now!"
He's also grown up a lot.
After his record-making turn in 2008, the young swimmer's life transformed, and for a while, Phelps was making headlines more often for partying than his performance in the pool. But the mistakes helped him grow.
"Michael's always been a role model," says Debbie, who was recently in New York City speaking on behalf of The Century Council to promote Ask, Listen, Learn, an alcohol education program geared towards middle school-aged kids and their parents.
"I've heard Michael talk about his successes, and when he fell on his face," she says. "We all go through life lessons; it's about how you embrace them and go from there."
Difficult for MomFor Debbie – who, with her two older daughters Hilary, 34, and Whitney, 32, has become famous for tearfully cheering on her son from the bleachers at the Games – watching Phelps step down from the podium that final time will be especially hard.
Starting when Whitney was 7, the single mom was shuttling her kids to and from twice-a-day practices, sacrificing holidays and vacations so they could pursue their Olympic dreams.
"It's not just the end of Michael swimming, but the end of a 25-year era that the Phelps family has been involved with," Hilary tells PEOPLE. "And it's culminating with Michael essentially being the greatest Olympian of all time – so there's a nice bow on it!"
"It's just about capturing memories. This whole year has been about that," adds Debbie. "All these lasts are bundling up."