"It hit its worst at the French Open. Every day I was in my room and I was online," she said. "I wasn't able to stop and I bought, bought, bought, bought, bought."
Her favorite items were clothing, shoes and things for her dog.
"Things I didn't need. I didn't even wear them," she said, laughing. "I was just out of control."
Williams said she was shopping online three hours daily during the tournament and up to six hours on off days. She initially got hooked as a way to avoid shopping in public.
"People always recognize me no matter where I go," said Williams, who is a platinum blonde now. "That's why I changed the color of my hair, so people wouldn't recognize me as much. But it didn't do any good, so I was like, OK, I'll try the online thing and next thing I know, it's fun and I was addicted."
Williams says she's better now, although she is tempted whenever catalogs arrive in the mail.
"I throw all catalogs out now," she said.
Williams, 19, has earned more than $700,000 on the court this year. The Olympic champion also has a $ 12 million deal with Puma, a sports apparel manufacturer. Together with older sister Venus, she has attracted new attention to the sport of tennis.
© MMI, CBS Worldwide Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report