Petra Kvitova and Agnieszka Radwanska are the latest players to suffer setbacks before the Australian Open.
Kvitova, 25 will not defend her title at the Sydney International, a warm-up event to the year's first Grand Slam, because of a stomach illness.
Second seed Radwanska, 26, has also withdrawn because of a leg injury.
The world's top-four ranked players - Serena Williams, Maria Sharapova, Simona Halep and Garbine Muguruza - have also had injuries this week.
World number six Kvitova, seeded third for the Sydney tournament, also withdrew from the Shenzhen Open in China last week because of her stomach virus.