The Irish phenom is the crown jewel of Nike Golf and the apparent replacement of Tiger Woods in its marketing effort; he earns a reported $20 million a year in endorsement fees to rep the Swoosh from head to toe when he’s on the course. But he didn’t play at the British Open last month, after having ruptured an ankle ligament while playing soccer for fun.

McIlroy’s absence from the Open meant a Major tournament at which Under Armour enjoyed unchecked exposure. Its own golf star, the quiet Texan Jordan Spieth, was the talk of television and brought eyeballs to Under Armour’s logo every time he teed up. Under Armour signed Spieth to a reported 10-year, $200 million deal (likely equivalent to, if not a bit more than, McIlroy’s deal) in January. Its sponsorship of Spieth has unarguably been a smash success: since signing the extension (he first signed with Under Armour in 2012), he has won The Masters (the same week Under Armour launched its first golf shoe—a corporation cannot buy or hope for such magical timing), won the U.S. Open, and come inches shy of making a playoff that would have put him in contention to win the British Open.

Some pundits feared McIlroy would be out for the rest of the year. Instead, he announced on Monday that he would play in the PGA Championship, which begins on Thursday at Whistling Straits in Wisconsin. He told reporters that his ankle is ready to go: “To do what I need to do this week, it’s 100 percent,” he said. Whether he is fully healthy or not, his presence will bring attention back to Nike. His presence will make it a Nike(NKE) vs. Under Armour(UA) brand battle on the course. McIlroy and Spieth are even paired together for the tournament, so viewers can expect shot after shot of the two of them walking side-by-side, logos front and center.

The two sports apparel giants have a similar strategy in golf endorsements: bet big on one or two horses. For now, Spieth is Under Armour’s stud (though it also sponsors Hunter Mahan); Nike’s are McIlroy and Tiger Woods (no other Nike golfer is a household name). Woods will be at the PGA Championship, but missed the cut at the last two Majors and has not won a Major since 2008; he is not expected to impress. McIlroy has won four Majors, Spieth two—but Spieth’s wins were both this year. He is on fire, and if he wins or places second at the PGA Championship and McIlroy places third or worse, Spieth will take McIlroy’s current No. 1 ranking.

Adidas, meanwhile, sponsors a stable that includes quite a few big names, like Jason Day, Dustin Johnson, and Sergio Garcia — all of whom will play this week. But unlike Nike and Under Armour, Adidas doesn’t sign any to “lifetime” deals worth hundreds of millions, and it doesn’t deck them out head-to-toe. (And last week, Adidas revealed that it is looking to sell off parts of its golf business; this may be why it hasn’t invested much in promoting its golfers.) Equipment maker Titleist boasts the largest roster of top golfers, but blends in on the course a bit more since fans are used to seeing it. The four Majors every year are crucial opportunities for the broader sports apparel brands to associate themselves with golf.

Even if Spieth has the better chance of winning this week, Under Armour has the bigger challenge. According to data that social analytics firm Networked Insights shared with Fortune, Nike got the lion’s share of social media mentions during the British Open—without Woods or McIlroy competing. Of course, there are other ways to measure brand impact than social media, but it is a measurement of increasing importance, and the data was a reminder that Nike is the dominant force in sports mindshare. (Nike also won the social media battle at the Women’s World Cup, even though Adidas was official sponsor of the event.)

Under Armour, after all, is a relative newcomer to golf. It aims to build a golf line on the back of Spieth, much like Nike did with Woods. But winning the brand battle at the PGA Championship will be a tall task.