Six-times Grand Slam champion Serena Williams has signed an endorsement contract with sportswear manufacturer Nike that could make her the richest woman in sport.

"We are extremely pleased that Serena will be joining the team of Nike athletes," said Riccardo Colombini, global director of the brand's tennis marketing division, in a statement on Thursday.

No details of the agreement were made public, but sources close to the deal said it is an initial five-year link with the option of a further three years.

If Williams meets performance-related bonuses over the full eight years she could net close to the $55-60 million figure quoted in media reports.

That would overhaul the $38 million, five-year agreement between her sister Venus and Reebok and almost certainly surpass the massive endorsement earnings of Russian Anna Kournikova.

Serena, the 22-year-old whose game was developed on the public courts of Compton, Los Angeles, has not played competitively since capturing her second Wimbledon title in July and her world ranking has slipped from first to third.

A knee injury, which required surgery, has kept her sidelined, although she is expected to return to action at this month's Hopman Cup in Perth, Australia.

Williams, who become well-known for her unique tennis outfits made by former sponsor Puma, has talked previously of her desire to pursue other interests outside the sport though she has said she plays to continue playing for another 10 years.

"She is one of the greatest athletes, male or female, in the world," said Nike spokesman Dave Mingey.

"We cannot reveal specifics (about how involved Williams will become in the design), but we will be working with her as both a woman and an athlete."