World number one Ashleigh Barty has withdrawn from Indian Wells and the Miami Open, saying her body has not fully recovered from her Australian Open triumph.

Barty was the two-time defending champion in Miami.

"I don't believe I am at the level necessary to win these events," the 25-year-old said in a statement.

Barty has had an unbeaten start to the season, winning the Adelaide Open before becoming the first home women's Australian Open champion for 44 years.

She said she hopes to return for Australia's Billie Jean King Cup qualifying event against Slovakia on 15 April.

"Unfortunately my body has not recovered the way I'd hoped after the Australian Open and I have not been able to adequately prepare for Indian Wells and Miami," Barty said.

"I love these events and am sad not to be there competing but getting my body right must be my focus."

Melbourne finalist Collins has also withdrawn from the two American hard-court events.

Indian Wells takes place from 9-20 March, with the Miami Open following between 21 March-3 April.