Molik, 27, entered the elite rankings in 2005 after winning three WTA events and the Olympic bronze medal in 2004. But an inner ear infection in the spring of 2005 affected Molik's vision and balance and kept her off the court for about a year.
Additional injuries hampered Molik's attempts to regain her previously high ranking. She is currently ranked 143rd.
"It's tiring and it's very draining but I guess it's a number of things culminating," Molik said.
"It took a while to get back, and since then I've just struggled with a few more things that I never imagined would come along the way. But that isn't the only thing. I think I'm still young enough to focus my energies on something that I feel is again challenging."
Molik won five WTA tournaments as a singles player and seven more, including the 2005 Australia Open and 2007 French Open, as a doubles player.