As the National Football League begins another season, almost a third of its starting quarterbacks — including Peyton Manning and Drew Brees — will be represented by Creative Artists Agency. And as Novak Djokovic won the U.S. Open on Sunday, his team from William Morris Endeavor was in the stands cheering him on.

Bitter rivals in Hollywood, William Morris Endeavor and Creative Artists, the nation’s two largest talent agencies, are going all in on sports. With the movie and television businesses facing an uncertain future — cable networks are losing subscribers, advertising is moving online, digital movie sales are not yet the new DVD — both agencies have turned to athletics, one area of the entertainment industry that continues to grow unabated.

Live sports programming has always commanded billions of dollars from networks and advertisers, but the value of sports media rights has skyrocketed as distribution opportunities have increased and ratings for dramas and comedies have eroded.

Movie and television stars are also having a harder time capturing mass consumer attention, but the value of athletes — not just as performers but also as marketers — is growing. The accounting firm PricewaterhouseCoopers estimates world sports revenue at $145 billion for 2015, almost 60 percent more than estimated filmed-entertainment sales (which does not include some forms of television revenue) of $88 billion.

“These two agencies used to play in a Hollywood silo, but they looked across the media landscape and saw sports gaining outsized importance,” said Chris Bevilacqua, a former Creative Artists executive and a co-founder of Bevilacqua Helfant Ventures, a sports and media advisory firm. “If size is a weapon, sports is the area of entertainment that is growing the fastest.”

Creative Artists began to build a sports operation in 2006 with the hiring of several prominent sports agents. Today, it remains a major player in representing athletes in the NFL and other professional leagues. But it also has pushed into an array of other sports-related businesses — representing teams and marketers in lucrative corporate sponsorships and advising on the sales and purchases of teams such as the Philadelphia 76ers and Texas Rangers.

But sports really took center stage in Hollywood in 2013, when IMG — a storied agency that first became known for representing golfers and tennis players — went on the auction block after the death of its owner, Theodore J. Forstmann, in 2011. Creative Artists was interested, but William Morris’ co-chief executives, Ari Emanuel and Patrick Whitesell, backed by the private equity firm Silver Lake Partners, emerged victorious, paying a hefty $2.4 billion for what is now a media and entertainment giant.

William Morris Endeavor and Creative Artists keep close tabs on each other, even though they profess otherwise. (“I kind of think about them a third of the time-ish,” Emanuel said of Creative Artists at a tech conference in July. A spokesman later said even that was a joke.) And now that both agencies are majority-owned by private equity firms (Creative Artists is controlled by TPG Capital), they are looking to expand and diversify their businesses, presumably in pursuit of the same end: a sale or an initial public offering.

And they are not above some competitive sniping.

“We did not need to, nor choose to, spend billions of dollars to acquire a company built in a time of different business realities,” Richard Lovett, president of Creative Artists, said in an email. “So, rather than being burdened with debt, struggling for growth and looking for an exit, we have a clear plan, a business focused on the future, and the best leadership in the business.”

William Morris Endeavor declined to comment.

Although the sports units bump up against each other, they are employing different strategies. Creative Artists pursues players in team sports (the basketball player Dwyane Wade, soccer star Cristiano Ronaldo) and William Morris Endeavor focuses on athletes with more individual careers (sprinter Usain Bolt, the mixed martial arts star Ronda Rousey). William Morris Endeavor owns sporting events and distributes more than 30,000 hours of sports programming annually, while Creative Artists has decided that those businesses carry too much overhead.

The agencies’ sports businesses also differ in terms of size. Creative Artists has about 400 employees in the sports business. William Morris Endeavor’s IMG has nearly 5,000 workers globally, most of them employed in labor-intensive college sports and broadcast and event production businesses. Last year, IMG made about $1.5 billion in sports revenue, according to people briefed on its financials. Creative Artists, which is similarly private and does not disclose its numbers, had sports revenue of less than one-third that amount, people briefed on its performance said.

The agencies’ balance sheets look different as well: Creative Artists has less debt, having taken a slower and steadier growth trajectory, while William Morris Endeavor borrowed a great deal to buy IMG. And Emanuel and Whitesell continue to push William Morris Endeavor into new arenas. In April, the agency paid about $100 million to acquire the Professional Bull Riders circuit. In a statement at the time, Whitesell and Emanuel said they had a “passion for expanding the sport of bull riding globally.”

IMG also negotiates and oversees big marketing deals, including a longstanding relationship between Visa — one of the world’s largest credit card companies — and the FIFA soccer organization, and between General Electric and the Olympics. Although it has largely eschewed representation in team sports, IMG has clients in other sports, including the tennis player Maria Sharapova, who have been among top earners of endorsement income.

College sports is emerging as a battleground. IMG College is one of IMG’s biggest divisions, and in March, it and the NCAA struck a 10-year deal — which includes ticket sales and marketing for certain championship tournaments — that deepened their relationship. IMG already has a lucrative licensing unit through its Collegiate Licensing Co., which manages the brands for about 200 university and college teams.

Creative Artists has started to encroach on the upper end of the licensing business with its acquisition in April of the Atlanta-based Fermata Partners, a company started in 2012 by former executives of IMG’s Collegiate Licensing Co. Fermata has signed up a number of high-profile institutions, including the University of Georgia, the University of Kentucky and the University of Miami — all three former clients of IMG College. (A William Morris Endeavor spokesman noted that IMG College had recently added new clients including Michigan State University and Northeastern.)