Williams said in a telephone interview that he got a call from Under Armour the day after the NBA draft. He also talked to Nike, he said, but felt he might receive better exposure with Under Armour. He also said he liked the fact that Under Armour had signed a number of younger players to build its basketball line.
As it has become more profitable, the company has been able to target a few better-known athletes.
Hometown hero Michael Phelps signed with Under Armour last year — the Olympic swimming champion agreed to wear the company's products outside the pool.
"For Michael, this was really all about finding a way to work with a brand he loves," said his agent Peter Carlisle. "Michael and Under Armour both hail from Baltimore, which makes it a natural fit; but what really drives it is his affinity for Under Armour, its brand, products and people."
The company also signed last year what it called its highest-profile athlete ever, NFL quarterback Tom Brady. The agreement gave the New England Patriots player equity in the company to help seal the deal.
"They've been aggressive for guys with smaller checkbooks than their peers," said Michael Binetti, an analyst with UBS Investment Bank.
The endorsement issue came up during a recent event for investors at Under Armour's Tide Point headquarters. Plank said endorsements are important but said the company picked them strategically.
"We don't believe we have to pay a ransom for it," he said at the time.
CEO Plank often calls athletes personally to persuade them to join the company. He also looks for athletes who want to grow along with the firm.
"Other companies have so many athletes that they sit around for years with hopes of being marketed," Mirchin said. "If they want to pick up a check and not build a brand, that is not the person we are looking for."
Newton, who will play for the Carolina Panthers this season, said at a recent Under Armour shareholders meeting that he chose Under Armour over Reebok and others because he likes the direction it's taking.
"Y'all have seen just the appetizer of where this company is going to go," Newton said.
Under Armour's strategy of generally hiring less-than-megastar athletes is not without risks. Rookie players don't always live up to their potential. Younger players may not be as mature as older players and more likely to get into trouble off the field, sports marketing experts say.
"They could identify people who never make it," said Amna Kirmani, professor of marketing at the Robert H. Smith School of Business at the University of Maryland. "There are plenty of rising stars who don't make it as professional athletes."
Mirchin said the company is aware of the risks but said it chooses athletes carefully. Under Armour has a team that follows young athletes throughout their college careers.
"Every athlete doesn't have to succeed," Kirmani said. "If they get one or two [who] really make it, that will benefit the company."