SYDNEY - Defending champion Petra Kvitova and second seed Agnieszka Radwanska are the latest stars to have troubled buildups to this month's Australian Open after pulling out of the Sydney International on Sunday.

Third-seeded Czech Kvitova will not be able to defend her Sydney title after succumbing to a gastrointestinal illness, which plagued her in China last week.

Poland's Radwanska joined Kvitova after withdrawing from the Sydney tournament with a left leg injury. Their withdrawals from the final lead-up tournament to the year's first grand slam event came after injury problems at other Australian tournaments for the world's top-four ranked stars Serena Williams (knee), Simona Halep (Achilles), Garbine Muguruza (foot) and Maria Sharapova (forearm).

"I'm really disappointed to have to withdraw from the Sydney International but unfortunately I am still not feeling well," Kvitova said. It is the second consecutive event from which the world number six has withdrawn in the new year. She retired with a stomach virus during the first round of the Shenzhen Open in China last week.

Radwanska said she needed to rest her leg after playing five matches in the past week. "I'm really sorry that I can't play in Sydney this week. I am feeling very confident in my game at the moment and the results are there, but after five matches in the past week my leg needs some recovery time," she said.

In Sunday's matches, 2014 champion Bulgarian Tsvetana Pironkova upset Lesia Tsurenko 7-6 (8/6) 6-2 to set up a second-round encounter with Swiss star Belinda Bencic. Serbia's Jelena Jankovic accounted for American Coco Vandeweghe in straight sets and Caroline Garcia won the all-French match with Kristina Mladenovic 7-6 (7/4), 6-4.

Serbia's world number 16 Ana Ivanovic, who was granted wild card entry into the main draw, was knocked out by Czech Karolina Pliskova 6-4, 6-2. It was her second first-round exit in as many weeks after losing to Britain's world number 122 Naomi Brady in Brisbane last week. The Australian Open gets underway in Melbourne on January 18.