Russians Svetlana Kuznetsova and Nadia Petrova both withdrew from the Sydney International on Tuesday.
Kuznetsova is a former US Open champion
World number four Kuznetsova pulled out of her match with Katarina Srebotnik complaining of breathing problems, having lost the opening set 6-2.
She said she was suffering from an upper respiratory tract infection but is confident of being fit for the Australian Open.
Fourth seed Petrova withdrew with an abdominal strain against Ana Ivanovic.
She was trailing 6-4 4-2 at the time.
Kuznetsova, the former US Open champion, said she withdrew to give herself a chance of playing in Melbourne next week.
"This really disappoints me a lot," she said. "But I just need to stop in the middle of the rally to breathe, and my nose is closed and I hear everything ringing in my ears.
"The blood is going up and I can't really focus with everything else."
"I definitely will play the Australian Open but if I feel a bit like this I will push myself and maybe I will have to retire again," she added.
"I am really motivated to play. But if I am the same way as I am today, I won't. But if I get better, I think I should.
"It's impossible I stay like this one more week. It's not that serious for me."
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Top seed Amelie Mauresmo and Kim Clijsters both won to reach the quarter-finals.
Mauresmo, who will be defending her Australian Open crown next week, was made to battle by fellow Frenchwoman Tatiana Golovin.
Mauresmo hit 43 unforced errors but finally came through the second-round match 6-7 (4-7) 7-5 6-1.
Things were much easier for third seed Clijsters, who eased past Australian Nicole Pratt 6-1 6-2.
Jelena Jankovic, who upset Martina Hingis in the opening round, continued her progress with a 6-2 7-6 (7-2) win over Samantha Stosur of Australia.