Former Wimbledon champion Maria Sharapova has reportedly extended her sponsorship agreement with Nike by eight years for $75 million.

The deal takes effect this month and includes a line of dresses designed by the former top-ranked tennis player.

The 22- year-old will also get a percentage of sales, according to the person, who asked not to be identified because the terms are private. Nike spokesman Derek Kent declined to comment.

Nike is the world's largest athletic-shoe maker, and has worked with the Russian for 11 years. Since winning Wimbledon in 2004 at the age of 17, Sharapova has become one of the biggest draws on the WTA Tour and the world's best-paid female athlete. She has also won the Australian and US Opens.

"Sharapova is one of those stars whose name transcends sports, similar to David Beckham," said Stefan Szymanski, an economics professor at the Cass Business School in London. "She's become an international celebrity first, and an athlete second."

Sharapova is the fourth favourite to win the Australian Open, which starts January 18.

Kim Clijsters and Serena Williams are co-favourites at 3-1, ahead of Justine Henin at 4-1, according to U.K. bookmaker Ladbrokes. Sharapova is 8-1, the gambling site said.

Sharapova makes close to $23.5 million a year in prize money and from endorsing companies including Tiffany & Co., Sony Ericsson Mobile Communications Ltd. and Canon Inc., according to Sports Illustrated. She was the only woman in the magazine's July list of the top 20 highest-earning non-U.S. athletes.

"She's very happy to stay with Nike, to stay with the family she's been with since she was 11," Max Eisenbud, Sharapova's agent at IMG Tennis, said in a telephone interview from Coral Gables, Florida.

Venus Williams extended an agreement with Reebok in 2000 that the clothing maker said at the time was "the most lucrative for a female athlete." The five-year contract was worth about $48 million, the player's family attorney said at the time.

Sharapova, who has nine sponsors, may drop some endorsements in favour of agreements that give her a percentage of sales, Eisenbud said in an interview in September.

"She has wealth," Eisenbud said at the US Open in New York. "She wants to focus on deals where she has equity, where she helps designing, gets a percentage of the sales."

Sharapova already had an equity agreement in place with Cole Haan, a wholly owned Nike subsidiary and US clothing, shoe, handbag and accessory designer.

The extension of the Nike deal comes less than a year after Sharapova returned from a right-shoulder injury that sidelined her for nine months and forced her to undergo surgery.

The injury led her to miss the 2008 Beijing Olympics, the 2008 US Open and the 2009 Australian Open.

Ranked outside the top 100, Sharapova returned to the WTA Tour in May.

She made the quarterfinals of the French Open - the only major she has yet to win - and then lost early at both Wimbledon and the US Open.