When Andy Roddick was at the height of his tennis career, he knew he had to be smart about his money.

So he started building his wealth instead of spending it and is now a successful real estate investor.

His advice to others: Start investing at an early age.

"The most powerful thing you can be is a young investor," the one-time U.S. Open champ said. "Time is your friend."

Roddick entered the world of professional tennis in 2000 at the age of 17. He won the U.S. Open when he was 21 and briefly held the title of No. 1 ranked player in the world.

At the time, a mentor said, "you don't have to wear your wealth," recalled Roddick, who earned $20.6 million in prize money during his tennis career.

Year-over-year returns are fine ... decade-over-decade returns are a lot better.

Andy Roddick

Former U.S. Open champion

So he paid close attention to his financial health and looked for ways to give back.

When he was just 18 years old, he started the Texas-based Andy Roddick Foundation. The nonprofit focuses on engaging children outside of their time in the classroom, helping them grow in subjects like science, art, sports and literacy including financial literacy.

He also invested wisely. During the 2008 financial crisis, he began snapping up real estate when people were liquidating. He was able to lock down 15- and 20-year leases, namely on bank buildings.

He now has about 70 properties, but said he isn't aggressively buying at the moment.

While Roddick acknowledges that he's been extremely fortunate, he said you don't have to have a fortune to start saving and investing.

"You talk to some young people [who say], 'Well, I'm only pulling in x amount,''' he said. "I'm like, 'Well, 20% is the same regardless of how much you have. You know, put something away every month and be disciplined about it.'"

PHILADELPHIA, PA - JULY 25: Tennis player Andy Roddick attends the 2017 Mylan World TeamTennis New York Empire vs Philadelphia Freedoms match at Michael J. Hagan Arena at St. Joseph's University on July 25, 2017 in Philadelphia, Pennsylvania

Gilbert Carrasquillo | Getty Images Entertainment | Getty Images

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Tennis legend Andy Roddick: A young investor is the 'most powerful thing you can be' - CNBC