World number one Justine Henin-Hardenne has pulled out of this month's Australian Open, organisers of the tennis event said on Thursday.

"At this stage we don't know the reason why but she is releasing a statement on Friday," a spokesman for the year's first grand slam tournament said.

The 24-year-old Belgian, who reached the final of all four grand slams in 2006 but won only in Paris, had earlier on Thursday withdrawn from the defence of her Sydney International title next week.

"I was told the withdrawal was for personal reasons," Sydney tournament director Craig Watson said in a statement.

"Obviously we are very disappointed, Justine is our defending champion and the world number one.

"I believe she is quite upset and we certainly wish her the best and hope she is able to return to the courts very soon."

Henin-Hardenne finished as year-end number one for a second time after winning the Tour Championships in Madrid in November. She pulled out of the deciding doubles match in the Fed Cup final against Italy in September due to a knee injury, handing victory to the Italians.