Nike Inc has top billing among sports brands at the Rio Olympics, but a week into the Games, its stock is losing to adidas AG and Under Armour Inc.

The quadrennial global sports event exposes apparel brands worn by top athletes to hundreds of millions of viewers, but growing competition and higher costs may be reducing the benefits of major endorsement deals.

Since the Summer Olympics started, shares of official games sponsor Nike had gained 1.0 percent as of Friday. But rival stocks have dashed ahead, with Under Armour up almost 3 percent and adidas nearly 6 percent, after a strong quarterly report.