That afternoon, Ms. Williams had escaped her busy schedule of training for the French Open and promoting charities to indulge in a shopping jaunt in New York City, accompanied by Nikki Burchiere, her personal assistant. Lunch was a sidewalk-vendor pretzel wolfed on the escalator at Barneys.

Ms. Williams, the reigning United States Open champion, was looking for jeans, yes, but also for inspiration for her new fashion line, Serena Williams Signature Statement, which she said she was active in designing. Her jewelry and handbags, all under $100, will be introduced this month on HSN.

Thumbing through the racks in Rag & Bone, Ms. Williams discussed her latest design theories. “Modern buildings can be really influential — the shape, the structure,” she said, pointing to the sleek, Le Corbusier-like geometry of a gray tunic dress. “I’m not saying this was influenced by a building, but it easily could be, these lines here.”

Fashion has long been a big part of Ms. Williams’s identity, as when she flummoxed tennis traditionalists by showing up for the United States Open in a black Lycra catsuit. Since then she has become a Cher of the tennis court, serving aces in pink hot pants, punky denim, even a studded leather jacket that looked more appropriate for a Judas Priest concert. “I’m a performer,” Ms. Williams said, by way of explanation.

But off court, “you wouldn’t believe how plain I am,” she said, wearing a simple white T-shirt and black leather jacket. “I’m a plain Jane.” (Ms. Burchiere felt compelled to point out that her boss owns more than 300 pairs of shoes.)

Fashion is more than a marketing vehicle for her, she said. It also provides a mental release from tennis, which is a 9-to-9 job for 11 months a year. A lot of her time is spent in hotel rooms watching reality TV and scribbling designs in her ever-present sketch pad, she said.

“I’m a loner on the tour,” Ms. Williams said. “When I was growing up, I idolized Monica Seles and Steffi Graf, and when you get on tour, you see that these people stick to themselves.”

Eventually, Ms. Williams disappeared into a dressing room with a stack of jeans. She emerged minutes later looking defeated. “Nothing fit,” she said, not even the ones with the promising labels that read “booty” and “curvy.”

“I guess I’ll just have to design my own,” she said."