Martina could become an active singles player again. She could even achieve a respectable ranking (i.e. get seeded at slams). Would that be enough to satisfy her competitive juices? Probably only short-term.
An athlete in his/her early to mid 40's can be as strong and fit and fast as most of his/her competitors. Even be better than most. Nolan Ryan threw no-hitters in his 40's.
But, here's the rub. The one thing you can't do as well in your 40's, no matter how fit you are, is recover. Whereas a 20-year-old could push themselves to the limit and come out the next day none the worse for it, a 40+ person is going to feel it big time.