Spieth already won The Masters and the U.S. Open this year, and if he captures the British Open this weekend, he will be the first golfer to win the first three majors in a row since Ben Hogan in 1953. He will also be one more Major away from winning a Grand Slam at just 22 years old. (Spieth’s birthday is this month, just before the early August PGA Championship tournament.)

That accomplishment would be a big coup for Under Armour, which is Spieth’s biggest endorser—and would likely bring Spieth a generous bonus as part of his 10-year deal that is rumored to be worth as much as $200 million. Sports endorsement contracts are typically structured to include bonus fees for certain milestones or achievements, and winning a Grand Slam is just about the biggest achievement there is on the PGA Tour. (Recall the controversy this year when the New York Yankees denied Alex Rodriguez a promised $6 million bonus for hitting 660 home runs, though that was from his team, not an endorser.) If Spieth were to win, Under Armour is likely to begin pushing a lot more advertising involving the young Texan phenomenon.

It’s also a tournament at which Nike is somewhat absent. Nike’s marquee golf star, No. 1-ranked Rory McIlroy, is sitting out the tournament after getting injured earlier this month playing recreational soccer—a blow for Nike, which gets big exposure when the Irish star is on the course, covered in Swoosh gear from head to toe. Meanwhile, Tiger Woods, the sport’s one-time top superstar, had a brutal performance at the U.S. Open and, while still endorsed by Nike, has taken a visible backseat to McIlroy among Nike’s golf stars. In last year’s Fortune/Sports Illustrated Fortunate 50 ranking of the highest-earning athletes, Woods fell behind Phil Mickelson. “The door is open for Under Armour this weekend,” says Bob Dorfman, sports marketing executive at Baker Street Advertising. “Tiger Woods and Nike has become a sort of sad situation. And who does Nike have instead of Tiger? It’s Rory, who’s not even playing in this tournament.”

Meanwhile, Adidas, which is not often associated with golf in the U.S. (although the German corporation owns TaylorMade, the top golf equipment maker), has branding potential as well at this year’s Open in current top contender Dustin Johnson. The American golfer, who has never won a Major (but tied for second at this very event in 2011), is Adidas’s biggest golf endorser. Last year, he was suspended for six months by the PGA Tour after a positive drug test. Now he is back and was among the top leaders after one day of play Thursday. Adidas also endorses Sergio Garcia, a Spaniard who tied for second at this tournament last year.

Nike built an entire golf business on the back of Tiger Woods. Under Armour, in an impressive bit of good luck, launched its first ever golf shoe the very week that Spieth won The Masters this year. The company doesn’t yet make golf equipment, but with Spieth as the face of its golf apparel, it could try. (And if you ask our sister publication SI Golf, Spieth has a great shot because he won the U.S. Open “without even playing his best golf,” which is a scary thought for his competition.)

Even if Spieth doesn’t end up completing a Grand Slam this year, adding aBritish Open title would set up huge anticipation for the PGA Championship in August.

“This will be a huge boost for Under Armour if he wins this one, just because it makes the next one massive,” says Dorfman. “The press and the attention around him, and the exposure for Under Armour around that, would be astronomical.”

For now, Under Armour (UA) has appeared to take a hands-off approach with Spieth, experts say. The company has not pushed him too heavily or taken up much of his time. But if he wins the British Open, and then completes a Grand Slam at the astounding age of 22, you can bet you’ll start to see him everywhere.