Serena Williams, who seemed to make a farewell to competitive tennis at the US Open, dropped a hint she might not be done on Wednesday by evoking a famous retirement U-turn. Williams, a 23-time Grand Slam champion, said on ABC's "Good Morning America" that she liked the example of 45-year-old NFL star quarterback Tom Brady, who retired in February only to say six weeks later he was coming back for a 23rd season. "You never know," Williams said. "I've been saying that I think Tom Brady started a really cool trend." Williams, however, didn't commit to following in the footsteps of the seven-time Super Bowl champion who led Tampa Bay to victory in last week's NFL season opener. Williams, who turns 41 on September 26, said she was "evolving" away from tennis before the US Open, where her third-round loss seemed to mark a final farewell to her amazing career.