US Open champ Spieth: 'Really fortunate' to win

Golf's new golden boy: Jordan Spieth

U.S. Open champion Jordan Spieth told CNBC on Tuesday he was tense after completing his final round, "not being able to control the outcome." Dustin Johnson putted for the possible win but hit a three-put par to lose the championship.

"I really felt for him," the 21-year-old Spieth said on CNBC "Fast Money: Halftime Report."

"It took a few minutes for it really to sink in that I won," Spieth said. "I'm just really fortunate to be on this side of it."

Jordan Spieth poses with the trophy after winning the 115th U.S. Open Championship at Chambers Bay on June 21, 2015 in University Place, Washington.
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With his victory Sunday, second-ranked Spieth swept golf's first two major championships this year. He became only the sixth male player to win the Masters and U.S. Open in the same year.

"It's just been ... a year that I will never forget," Spieth said.

Spieth's remarkable run has given sponsor Under Armour a boost. He has helped the apparel company carve a presence in golf since it signed him in 2013. Earlier this year, he signed a new contract that will keep him with Under Armour until 2025.

Read MoreUnder Armour's early bet on Masters champ Spieth pays off

Spieth declined to disclose the terms or whether he owns any of the company's surging stock. But he said the partnership has been a "dream" and he "certainly looks to help their brand grow."

Spieth has taken in nearly $8 million on the course this year, not including what he earns from sponsors. They also include AT&T, Rolex and Titleist.

With the win, Spieth inched closer to knocking off No. 1-ranked golfer and Nike athlete Rory McIlroy from the top spot.

Spieth said the win at U.S. Open venue Chambers Bay sets him up for success at next month's British Open at storied St. Andrews in Scotland. He noted, though, that preparation for the course will be "significantly different."