When I was playing tennis a few years back I actually ended up injuring my abductor hallucis during a game. I didn't know it at the time, I just thought that I had landed on my foot the wrong way and was suffering from a slight sprain.
The abductor hallucis is a long muscle that runs down the side of your foot, pretty much up towards your big toe. For myself I was lucky and only need the RICE treatment to make my foot feel better. The hardest part of treatment was not being able to walk on my foot.
My doctor warned me that if I walked with a strained abductor hallucis that I could injure it further and possibly need surgery. Needless to say, I stayed in bed.