Nike and Under Armour already compete for National Basketball Association players' endorsements. It's a market Nike has traditionally dominated.
Now, the apparel and footwear makers appear poised to compete for a particularly large prize -- sponsorship of the league as a whole.
The opening comes because Adidas has decided not to seek an extension of its apparel deal with the league. The agreement expires in 2017.
The German company said in a statement that "while we have enjoyed a successful long-term relationship with the league, we continually review our partner agreements to ensure they are meeting our investment and delivering on our brand and business needs."
Last year, Under Armour passed Adidas in combined apparel and footwear sales to become the second biggest sports brand in the United States.
League sponsorship would seem to be a natural fit for the Baltimore-based company. Like soccer, basketball is an international sport and an Olympic sport. Much more than the NFL and Major League Baseball, the NBA has players and fans from around the world.
Under Armour has been steadily increasing its global presence, partly by signing soccer clubs in Europe and South America.
Under Armour officials did not respond to queries about the NBA opening.
Under Armour's highest-profile basketball player, Stephen Curry of the Golden State Warriors, is having a standout season and is expected to be in consideration for the league's most valuable player.