Weeks after it signed Roger Federer to a $300 million endorsement deal, Uniqlo is explaining why a company not known for sports attire is making such a big commitment to a tennis player in the twilight of his career.

The Japanese apparel giant said the tennis player will provide input on product designs, appear in ads and dress in the brand’s clothes both during matches and at charity and other public events off the court. He will don their latest collaboration, a line of polo shirts and shorts, at the U.S. Open this month.

“The collaboration is much bigger than sports,” said John Jay, Uniqlo’s global creative director. “We develop high-performance clothes for everyone off the court, and we’d like to think the technology is good enough for an athlete.”

The deal will cost Uniqlo roughly $30 million a year for the next decade, raising questions around why the Japanese brand picked Mr. Federer as an ambassador. He just turned 37 and is widely considered to be near retirement, though he recently won his 20th Grand Slam title this year at the Australian Open.

Uniqlo is mostly known for basics, not sports attire. It previously outfitted tennis player Novak Djokovic, who is now with Lacoste.