U.S. tennis superstar Serena Williams has said she always had a plan for retirement and has been "ready for that day" for nearly a decade.

The 40-year-old veteran discussed her retirement plans in an interview with Entertainment Tonight last week but did not provide any specific date.

The 23-time Grand Slam winner has not played professionally since Wimbledon last year where she was forced to retire due to a freak injury.

Williams has been a constant presence in the WTA scene since her professional debut in 1995 and is now in the twilight of her career.

"I’m ready for that day, I’ve been preparing for that day for more than a decade. I think it’s very important to always have a plan and I’ve always had a plan," she said.

This has been evident over the last few years as the U.S. star turned focus away from tennis to family and entrepreneurship.

She welcomed her first child Alexis Olympia Ohanian Jr., with her husband Alexis Ohanian, in September 2017 and plans to extend the family.

"I definitely want to have more kids, it’s just like, balance is key, you know, and just trying to find that balance. I don’t know and the clock is ticking. I need to figure out when that will be and hopefully soon when I will feel no pressure," said Williams.