Carrying the hopes of cancer survivors he's heard from since going public with his diagnosis last month, Shanteau smoothly stroked his way through the four-lap race and qualified seventh in 2 minutes, 10.29 seconds.
"This was the longest day of my life," he said.
"That's why I'm glad finals are in the morning because you wake up, you race and you're not sitting around thinking about it all day."
Defying conventional medical wisdom, Shanteau decided to put off surgery to compete in the Olympics, but only after being checked repeatedly to make sure the cancer wasn't spreading and threatening his chances of a full recovery.
He found out he had testicular cancer just weeks before last month's U.S. trials, where he upset Brendan Hansen to earn a trip to China.
"It was weird walking on deck," Shanteau said.
"I was totally clear - not too many nerves, excited, but pretty calm. It almost didn't feel like the Olympic Games. That was probably good in a sense."
Shanteau and the rest of the world's top swimmers got the spotlight to themselves on a Michael Phelps-less night at the pool.
With Phelps off sleeping at the athletes' village ahead of two finals Wednesday, his teammates set him up to claim another gold medal by qualifying fastest in the 800 freestyle relay.
David Walters, Ricky Berens, Erik Vendt and Klete Keller combined for a time of 7 minutes, 4.66 seconds, lowering the old Olympic record of 7:07.05 set by Australia at the 2000 Sydney Games.
Their effort guaranteed lane 4 for Phelps and the rest of the relay team, which goes for its second consecutive Olympic title in an event the U.S. has dominated.
Standing behind the starting block for the 200 breast, looking down the length of the pool, Shanteau had a realization.
"You hype it up so much in your head and then you get out there and you're like, 'Wow, this is just another meet, I'm here to do the same thing. It's no different,'" he said.
"But at the same time that was still a dream come true for me just to be able to race in the games."
Daniel Gyurta of Hungary qualified quickest in 2:08.68, erasing Kosuke Kitajima's Olympic record from four years ago in Athens. Paolo Bossini of Italy was second at 2:08.98 and teammate Loris Facci was third.
Kitajima, the defending champion seeking a sweep of the breaststroke events in Beijing, qualified sixth-fastest in 2:08.89. American Scott Spann also moved on in 10th.
Fresh off his stunning anchor leg in the victorious 400-meter freestyle relay, American Jason Lezak advanced in the 100 free preliminaries.
Lezak has some work to do, though, if the 32-year-old American is going to make the 100 free final. He moved into the semifinals as 11th-fastest, with a time of 48.33 seconds.
That was considerably slower than the amazing 46.06 anchor leg he swam to lead the U.S. to a world record and gold medal in Monday's relay. He outtouched former world record-holder Alain Bernard of France at the wall, barely preserving Phelps' run at Mark Spitz's record of seven gold medals in a single Olympics.
Eamon Sullivan of Australia, who broke the world record on the relay's leadoff leg Monday, was fastest of all, at 47.80.
"I think emotionally it probably did take a little bit out of me, but I've been very careful about what I've been doing and reacting to that," Sullivan said.
Stefan Nystrand of Sweden was second-quickest in 47.83, followed by Canadian Brent Hayden (47.84) and Bernard (47.85).
Two-time defending champion Pieter van den Hoogenband of the Netherlands moved on in fifth with a time of 47.97.
The mostly Chinese fans at the Water Cube cheered overtime when Liu Zige turned in the fastest qualifying time in the women's 200 butterfly. She touched in 2:06.46, with teammate Jiao Liuyang fourth.
