DeJong, who was ranked fifth in the world in backstroke, had narrowly missed qualifying for the 2004 Olympics and badly wanted to make it to Beijing.
But he lost to Phelps by three-tenths of a second, effectively ending his Olympic dream.
I knew then that my competitive swimming career was over. I really didn't know what I would do next, said DeJong, who was 24 years old at the time. It was a difficult, confusing time for me.
Yet, it was DeJong's loss to Phelps that also cleared the way for him to find his next pursuit.
Today, DeJong runs the Big Blue Swim School, a business he co-founded in 2012 that offers swimming lessons for kids. The school currently operates five locations in and around Chicago. After turning the operation into a franchise last October, DeJong is hoping there will be as many as 400 locations nationwide in the next 10 years.
To be perfectly honest, I'm pretty sure if I had made the team, I would have not left the sport with the desire necessary to start a business, he said.
DeJong's love of swimming goes back to his childhood.
I grew up in a little town near Lake Michigan and would swim all the time, he said. He was already swimming competitively by the time he was eight years old.
After years of training, competing in college and then swimming professionally for a few years, I still never thought of swimming as a 'job,' said DeJong. It's just impossible to get out of the water in a worse mood than when you got into the water.
But after his competitive career was over, he struggled to figure out what was next. DeJong, who was an English major in college, thought law school was a possible next step.
It was a fleeting thought. He wasn't ready to turn his back on swimming.To help make ends meet, DeJong had been offering private swim lessons to three- to five-year-olds in the Chicago area. It was a rudimentary operation at first, and he mostly relied on word-of-mouth and his own reputation as an elite swimmer to attract new clients.
I would rent pool space wherever I could find it in the suburbs, he said.
He also enlisted the help of his friend John Lonergan, a competitive swimmer at the University of Iowa, and the two taught weekly swim lessons.
Each 30-minute lesson cost $20. For DeJong, teaching kids was a way to recover from the setback of his loss and rediscover the fun of swimming.
We did these lessons for two-and-a-half years and realized by then we had as many as 600 kids we were teaching weekly, said DeJong. Plus, we were making money.
The two formulated a business plan and sought investors, including some parents of students, to fund the construction of a permanent pool.
DeJong said parents were already fans of his teaching approach, which he had based on his own training. First, parents liked that we are teaching a vital life skill. And our approach was to teach a child to not just swim but to swim independently and across the pool in the first 10 lessons, he said.
One of the first investors to come on board was Chicago Cubs co-owner and Republican National Committee Finance Chairman Todd Ricketts, whose daughter was already DeJong's student.
Ricketts and a few other early investors together invested $1.2 million, which helped DeJong secure the location to build a permanent pool.In April 2012, DeJong and Lonergan opened their first Big Blue Swim School in Wilmette, Illinois, a suburb north of Chicago.
DeJong helped design of the facility and worked on the curriculum (for infants and children up to 12 years old).
In our pools, the water is always 90 degrees so kids don't feel cold. And we use antimicrobial carpeting on the floors to prevent falls and reduce noise, he said. The school also features an air conditioned viewing area with free Wi-Fi for parents.
The company also offered its own software called Lesson Buddy that lets parents schedule lessons and view their child's progress in real-time.
Just a week after opening, enrollment at the school jumped from 600 students. (who migrated over from the private lessons with DeJong and Lonergan and a few other instructors) to 2,200.
At first, DeJong said the crew was overwhelmed with the fast-paced growth, but they have managed to keep up and attract several new investors.
To date, Big Blue Swim School has raised a total of $20 million, including $16 million from Level 5 Capital Partners, which acquired a majority stake in the company and is a franchising partner.
[Big Blue Swim School is] tapping into unmet demand for this service. Financially it was a solid business and we especially liked that the business owns its own proprietary technology, said Scott Thompson, chief development officer with Level 5 Capital Partners.
While the kids are in the pool, there's also staff walking alongside watching and providing information and progress reports back to the parents in real-time via an iPad, said Thompson.
Since turning the business into a franchise last year, DeJong has closed deals on five franchised locations in Denver, 10 in Chicago, six in Minneapolis and three in Salt Lake City. He declined to disclose annual revenue but said the business is profitable.
According to the company, the total investment for a new franchisee ranges from $1.8 million to $3.6 million, including real estate and marketing costs.
While the steep initial investment may deter some potential franchisees, Thompson said it also acts as a barrier to entry to those who may not be the right fit.
Erik and Wendy Skaalerud plan to open four Big Blue Swim School locations in the Denver metro area in the next four years.
Swimming is a life safety issue. That's why we became involved, said Wendy. But we also like the overall teaching philosophy of building confidence through swimming and by setting goals.
For DeJong, the abrupt pivot from an athlete to a first-time entrepreneur without any prior business experience was challenging.
I describe it as building a plane as I'm flying it, he said.
But he attributes his success to an important lesson that swimming taught him.
Getting up after a disappointment is just as important in competitive swimming as it is in business, he said. Like with swimming, success in business is also measured by your perseverance.