Nine-time Grand Slam champion Serena Williams will play in January's Sydney International, organisers said on Thursday, meaning the event will feature the top three women players and the Olympic champion.

Williams, 27, the world number three and current US Open champion, joins world number one Jelena Jankovic, second-ranked Dinara Safina and Beijing gold medallist Elena Dementieva in the January 11-17 mixed tournament.

Headlining the men's event will be Australia's four-times champion Lleyton Hewitt, his Argentine rival David Nalbandian and Frenchmen Richard Gasquet and Jo-Wilfried Tsonga, the 2008 Australian Open runner-up.

"Having Serena Williams back for the first time in a few years is just an absolute boon for the event," tournament director Craig Watson said.

"Having Jo-Wilfried Tsonga back is really exciting as well."

Watson said he was keeping a few wildcards available and expects to have at least 15 or 16 of the top 20 women playing in the tournament.

Hewitt will be gunning for a record fifth Sydney title when he makes his comeback from hip surgery ahead of the January 19-February 1 Australian Open in Melbourne.