Defending women's champion Petra Kvitova has withdrawn from the Sydney International, marking the second consecutive week that the two-time Wimbledon winner has pulled out of tournaments due to a lingering stomach virus.

Kvitova retired during the first round of the Shenzhen Open in China last week. She was scheduled to play her first match in Sydney against Germany's Sabine Lisicki on Monday.

"I'm really disappointed to have to withdraw ... but unfortunately I am still not feeling well," the sixth-ranked Kvitova said in a statement Sunday.

Bernard Tomic is the top-seeded man at Sydney, while Simona Halep is the top-seeded woman.

In a first-round WTA match Sunday, Tsvetana Pironkova of Bulgaria defeated Lesia Tsurenko of Ukraine 7-6 (6), 6-2. Ana Ivanovic, who received a wild card, was scheduled to play her first-round match later Sunday against fifth-seeded Karolina Pliskova.

At the Hobart International, another of the Australian Open warm-up tournaments, seventh-seeded Alize Cornet of France beat Denisa Allertova of Czech Republic 6-4, 7-5 and Russian Margarita Gasparyan defeated 17-year-old Australian Maddison Inglis 6-4, 6-3.

Sloane Stephens, who won the ASB Classic in Auckland on Saturday, was a late withdrawal from Hobart due to a virus.

Last week, top-ranked Serena Williams, No. 2 Halep, No. 3 Garbine Muguruza and fourth-ranked Maria Sharapova all withdrew from matches due to injuries or illness.

The Australian Open begins Jan. 18 in Melbourne.