With the Australian Open just over a week away, Venus Williams and Sloane Stephens withdrew from warm-up tournaments on Sunday and another American, John Isner, could soon join them.
Williams, who lost the final of the Auckland WTA tournament on Saturday, pulled out of the Hobart International, informing officials on Sunday.
"I don't believe she has an injury," tournament director Mark Handley said. "She's played a lot of tough, close matches over in Auckland and in the lead-up to a first Grand Slam (of the season) she's not taking any chances."
Williams was planning to use the Hobart event as final preparations for the Australian Open which begins Jan. 13. She lost in the third round at Melbourne Park last year.
The seven-time Grand Slam champion's ranking has fallen to 47th as she has battled injuries and Sjogren's syndrome, a fatigue-causing autoimmune disease, over the past two years.
Stephens injured her left wrist at the Hopman Cup team event on Friday in Perth, and withdrew from her scheduled appearance at the Sydney International.
"I was having some pain in my wrist and the doctor recommended I not play this week," the 12th-ranked Stephens said.
Former No. 1 Caroline Wozniacki withdrew from the completed Brisbane International after hurting her right shoulder in practice, but is expected to play in Sydney, where she has drawn a qualifier in the first round.
Isner also appeared in doubt for an Auckland ATP tournament, where he is the third-seeded player, after re-injuring his right ankle at the Hopman Cup.
Gael Monfils also withdrew Sunday from the Heineken Open at Auckland, citing fatigue. Monfils lost in the final of the Qatar Open at Doha to Rafael Nadal on Saturday.
Jurgen Melzer left no doubt about his participation in the Australian Open, pulling out with a chronic left shoulder injury.
The 27th-ranked Austrian underwent surgery on a torn tendon in October and needs more time to recover, the Austria Press Agency reported on Saturday.
Melzer hasn't played since Shanghai last October.