Under Armour is expanding its empire

Under Armour has been aggressively investing in high-profile endorsement deals and building up its women's business in a bid to chip away at Nike's athletic empire.

The Baltimore-based company just announced a deal with boxing legend Muhammad Ali.

The deal comes just days after the company signed NBA All-Star Stephen Curry and announced the launch of its biggest global brand campaign to date, which will feature ads starring Curry and actor Jamie Foxx.

Under Armour's investments appear to be working,

The company's sales increased 32% to $3.08 billion in 2014, compared to the previous year.

Under Armour also recently surpassed Adidas to become the second biggest sportswear brand in the US by sales.

Best known for its performance wear, Under Armour has grown rapidly over the last 18 years. When it was founded in 1996, it had $17,000 in revenue. This year, it's expected to bring in $3.76 billion.

That's still only a fraction of Nike's sales, which are expected to total $28 billion this year. 

But Under Armour CEO Kevin Plank is confident his company can become a more formidable opponent to Nike.

That's why Under Armour has been increasing its investments in endorsement deals, an area where Nike has always excelled.

In addition to Curry and Ali, Under Armour also recently signed deals with supermodel Gisele Bundchen and ballerina Misty Copeland shortly after launching its largest-ever global women's marketing campaign. And early last year, Under Armour scored the biggest equipment deal in college sports. 

The company recently lost a bid for basketball star Kevin Durant to Nike, but Under Armour CEO Kevin Plank said he wasn't deterred by the failure.

"We're going to grow 20%-plus next year with or without the deal," Plank told Bloomberg TV's Stephanie Ruhle. "We wanted to send a message to every athletic director, to every president of every team club, to every league commissioner, that if you have a deal, there's no deal too big for us."