Phelps will also face a daunting opponent he can't see: the weight of great expectations fixed on his broad shoulders.
Advertising campaigns, a million-dollar bonus, national pride and a place in Olympic history are riding on his performance, which will be deemed a disappointment if it doesn't produce multiple gold medals.
Some athletes would flinch at the stressful circumstances.
"Thank God it's not me," fellow U.S. swimmer Natalie Coughlin said recently.
But others don't mind the pressure and expectations. Phelps, 19, says he loves it.
"I don't look at the attention as pressure. I look at it as support," Phelps said. His coach, Bob Bowman, said the comment was not just a bold pose.
"I think he likes being in the position of being expected to win, because that means you're the best," Bowman said. "Now, that doesn't mean you win every time, but that's the kind of person Michael is. He's a prime-time player. He wants the ball in his hands when he has to hit the shot to win the game."
The Rodgers Forge resident is taking aim at Mark Spitz's record of seven gold medals, set in 1972. Although he hasn't announced which combination of relays and individual events he will swim, he would be a heavy favorite in almost any individual event he would enter.
But as sports history has repeatedly shown, most recently in the NBA Finals last month, being a heavy favorite doesn't always guarantee success.
"In fact, it can be very debilitating in some respects," said Tom McMillen, the former Maryland Terrapins star who played on the U.S. Olympic basketball team that lost a controversial final to the Soviet Union in the 1972 Games.
"When you think you're supposed to win, that fear of failure can become part of the equation," McMillen said. "It shouldn't be, but when you go in with the odds stacked in your favor, it makes you think, 'What if I lose?' You think about those things. It's part of the mental process."
The Los Angeles Lakers' loss to the Detroit Pistons in the NBA Finals last month was a stunner, though not quite on par with other famous defeats, such as the Baltimore Colts' loss to the New York Jets in Super Bowl III, Georgetown's loss to Villanova in the 1985 Final Four and the Soviet Union's loss to the United States in the 1980 hockey "Miracle."
Did the pressure of being expected to win contribute to those defeats? Former Colt Tom Matte, who played in Super Bowl III, said that loss was more about being outcoached, but added "that kind of fear [of failure] can creep into your thoughts, no question. You do think about it."
Some athletes don't. Michael Jordan kept winning NBA championships with the Chicago Bulls when everyone expected no less. So did John Wooden's UCLA basketball teams and the New York Yankees of the 1940s and 1950s.
"It's really the fundamental test of greatness, the ability to face that kind of pressure and overcome it," McMillen said. "Those that can, that's why they're the Michael Jordans of the world."