US Open champion Bianca Andreescu has withdrawn from an Australian Open warm-up event due to injury.

The Canadian world No.5 had been the top seed for the Auckland Classic with organisers confirming the major setback to their tournament line-up.

“Organisers have been advised that Bianca Andreescu has had to withdraw from the upcoming event due to her ongoing knee issues,” the tournament said in a statement on Tuesday.

“We are disappointed not to welcome Bianca back and wish her all the best for a speedy recovery.”

The Auckland event from January 6-12 is traditionally used as key preparation for the opening grand slam event of the year at Melbourne Park.

The Australian Open begins on January 20.

Starting the year ranked world No. 178, Andreescu came through qualifying in Auckland earlier this year only to lose to Germany’s Julia Goerges in the final, which she used as a springboard for a sensational 2019.

She won her first WTA Tour title at Indian Wells, then beat Serena Williams in the finals in Toronto and again at Flushing Meadows to clinch her maiden grand slam title.

The Auckland tournament had assembled arguably its strongest ever field with 19-year-old Andreescu, Williams, former Australian Open champion Caroline Wozniacki and 2017 French Open winner Jelena Ostapenko all confirmed.

Two-times grand slam winner Svetlana Kuznetsova and American teenagers Amanda Anisimova and Coco Gauff have also entered the tournament.