Billie Jean King insists Martina Navratilova is "as brave as she is strong" after the latter was diagnosed with throat and breast cancer.

Navratilova is one of the most decorated players of all time, having won 18 grand slam titles during a glittering career.

The 66-year-old underwent successful treatment for breast cancer in 2010, but it has now been confirmed she is battling the illness again.

Navratilova said the diagnosis is "serious but still fixable," while a representative confirmed she will start treatment later this month and subsequently not attend the Australian Open.

King, a 12-time major winner, offered her support.

"Martina is as brave as she is strong," the 79-year-old tweeted.

"She has fought this battle before, and she is in our thoughts and prayers."