SYDNEY (AFP) - German world No. 7 Angelique Kerber withdrew from the Sydney International tennis tournament on Tuesday, adding to a lengthy list of ailing women stars ahead of next week's Australian Open.

The fourth seed, who won her first-round match against Ukraine's Elina Svitolina in fierce heat over three sets on Monday, pulled out citing a gastrointestinal illness.

Her withdrawal follows that of defending champion Petra Kvitova, who also had gastrointestinal problems, and second seed Agnieszka Radwanska, who dropped out with a leg injury.

Kerber, who lost to Victoria Azarenka in Saturday's Brisbane International final, said she experienced problems overnight after Monday's victory.

"I'm very sorry I can't go on court today but I'm not feeling 100 per cent," she said.

"I started to feel unwell yesterday and today my stomach is not right. I don't want to risk my health before Melbourne."

The trio's withdrawals came after injury problems at other Australian tournaments for the world's top-ranked stars including Serena Williams (knee), Garbine Muguruza (foot) and Maria Sharapova (forearm).

Russian Ekaterina Makarova received a walkover into the Sydney quarter-finals following Kerber's withdrawal.

The Australian Open starts in Melbourne on Monday.