Andy Murray: Under Armour's new weapon vs. Nike

Nike has long dominated tennis with many of the biggest stars in the game, but Under Armour is betting big on its newest star, No. 3-ranked Andy Murray.

Andy Murray of Britain hits the ball to Nick Kyrgios of Australia during their first round match at the U.S. Open Championships tennis tournament in New York, September 1, 2015.
Carlo Allegri | Reuters
Andy Murray of Britain hits the ball to Nick Kyrgios of Australia during their first round match at the U.S. Open Championships tennis tournament in New York, September 1, 2015.

Baltimore-based Under Armour added the 28-year-old to a four-year kit deal in January. Murray previously was an Adidas athlete.

"It's really based on the performance aspect. That's one of the main differences I've noticed since working with Under Armour," Murray told CNBC.

As Under Armour grows into an international brand, Murray brings brand exposure in a new market—Europe.

"It's a great way to show off your brand and how you do things," said Sam Poser, analyst at Sterne Agee CRT.

Read MoreThe world's 10 wealthiest tennis stars

The development of an Under Armour tennis sneaker, which would mark an even bigger bet on the sport, has so far been difficult. Murray has spent the last eight months testing its fit and style on different surfaces.

"The grass court shoes were perfect, but the hard court shoes are not quite there yet," he said.

Depending on how testing goes this week, Murray said he could be wearing his custom Under Armour shoes in time for the U.S. Open.

Since switching brands, Murray's ranking has risen from sixth in the world to third. Under Armour also has an endorsement deal with 29th-ranked women's tennis star Sloane Stephens.

Nike has long been the leader on the tennis courts. The Oregon-based brand represents many of the game's biggest stars, from Roger Federer and Rafael Nadal to Serena Williams and Maria Sharapova.

Sharapova, who said she's been with the brand since she was 11 years old, has seen the breadth of Nike's tennis gear evolve. "When I started, I had to cut my T-shirts and roll up my shirts. A junior line never even existed," she said.

Adidas' lineup includes many of the top women, including Ana Ivanovic, Caroline Wozniacki and Simona Halep. On the men's side, they sponsor French star Jo-Wilfried Tsonga and American Jack Sock.

Read MoreCheck out Rafa Nadal's new $775,000 watch

The No. 1 men's tennis player in the world, Novak Djokovic, is sponsored by an unlikely name—the Japanese company Uniqlo. His sponsorship has helped raise that firm's profile globally.

Analysts say that tennis footwear and apparel in the U.S. is a tiny market, and one that's in decline. Sterne Agee CRT's Poser said one of the reasons for the decline is that tennis apparel hasn't been fashionable off the court since the 1990s, when Andre Agassi played.

That said, brands continue to invest in tennis. "The brands want to show they have great athletes and great winners," said Poser.

According to Poser, the real value of sponsorships for apparel makers comes in the association with a winner.

"If you see the swoosh all day, you are more likely to buy Nike," he said.

Correction: Sam Poser is an analyst at Sterne Agee CRT. An earlier version misstated the name of the firm.