Top-ranked Victoria Azarenka of Belarus has withdrawn from her Fed Cup match against the United States because of a lower back injury.

The Australian Open champion was scheduled to play Christina McHale in Saturday's opening singles of the second-tier series. She was replaced by Anastasiya Yakimova but remains eligible to play Sunday in the best-of-five competition.

Azarenka says her lower back has been sore since the Australian Open. She says the long flight from Australia didn't help her back and she has had little time for practice.

Azarenka took the No. 1 ranking after winning her first Grand Slam title last weekend.