Williams, winner of last year's Australian Open, will miss the United States' opening match at the Hopman Cup in Perth, organizers said today.

The American star is trying to recover from a lingering left knee injury that forced her to pull out of the WTA Tour's season-ending championships in Madrid in November.

She had been scheduled to take on India's Sania Mirza on Saturday in the first match of the team event, but will be replaced by Meghan Shaughnessy.

Tournament officials said Williams will not arrive in the Western Australia state capital until the weekend, and plans to play for the Americans in Monday's match against the Czech Republic. Player substitutions are allowed between matches.

After an injury-filled 2006, Williams had a surprising win at the 2007 Australian Open — unseeded, ranked No. 81 and beating six seeded players en route to the title. By the end of the season, she was ranked No. 7.

Williams first injured her left knee in 2000 and had surgery in 2003, forcing her to pull out of all events after Wimbledon that year.

Jelena Jankovic and Novak Djokovic lead the top-seeded Serbia team at the eight-nation Hopman Cup that kicks off nine Down Under tournaments in two weeks leading into the Australian Open, which begins Jan. 14.

The ATP Tour has two tournaments scheduled outside Australia next week — in Doha, Qatar and Chennai, India, where Rafael Nadal is entered.

In Australia, a men's tournament also is scheduled in Adelaide, while the WTA Tour has tournaments in Auckland, New Zealand and on the Gold Coast in Queensland state, where Amelie Mauresmo is seeded No. 1.

During the week of Jan. 7-12, there's a men's tournament in Auckland and a women's event in Hobart, Tasmania. The Sydney International features Justine Henin as its top women's seed — where the Belgian will attempt to win her 40th career title — while James Blake and Lleyton Hewitt headline the men's field.

Defending Australian Open champion Roger Federer makes his first appearance of the year at an eight-man exhibition tournament at Kooyong in Melbourne beginning Jan. 9.

Also in the field are Andy Roddick, Fernando Gonzalez, David Nalbandian, Andy Murray, Tommy Haas, Ivan Ljubicic and Marat Safin.

Federer will aim to put the heat on his rivals in 2008, as he did in 2007 by winning eight tournaments, including three of four Grand Slams — his fifth straight Wimbledon and fourth consecutive U.S. Open — and finishing with a 67-9 match record.

While Federer provided many of the highlights in the 2007 season, gambling and concerns over match-fixing created negative headlines.

The ATP's suspension of Potito Starace and Daniele Bracciali last week followed a series of revelations by players that they had been approached with offers to fix matches, while there is an ongoing investigation into unusual betting patterns on a match between Nikolay Davydenko and Martin Vassallo Arguello in August.

The Australian Open has responded with an anti-corruption program that includes a former homicide detective as its chief investigations officer and a tournament hotline where players and fans can report incidences of corruption.

It also includes a ban on unauthorized use of laptops courtside, a block on gambling Web sites on publicly accessible computers at the tournament and increased security and restrictions to player access.