Novak Djokovic on Saturday pulled out of the upcoming Adelaide International in a big blow to the new tournament.
The world number two has been leading Serbia at the ATP Cup team event in Sydney, where they face Russia in a semi-final on Saturday.
No explanation was given by officials who confirmed his absence from the ATP-WTA tournament, whose women's field is headed by top-ranked Ashleigh Barty.
"We know how disappointed Novak is about not being able to play in Adelaide this year and we hope to welcome him to the tournament next year," tournament director Alistair MacDonald said.
"We understand his decision and wish him the very best of luck for the remainder of the ATP Cup and the upcoming Australian Open."