Post 99 is absolutely correct.
If your first serve is soooo weak that it get's returned by a girl, at 90 mph with strong topspin, you cannot possibly win a game against her!
But if you first serve solicits a desperate, reaching, lunging return, you can pummell the next ball into a corner, get to net position, and volley/overhead off the next shot for a winner.
You guys who say you can't beat a girl just don't have winner shots, forcing shots, or strong shots! If you did, you'd know that if you hit your best shot, that works against 5.0 men, it's also good enough against 6.0 women.
A strong 4.0, who's rising in rankings, has such shots.
A weak 4.0, who will stay there forever, or slip backwards, IS LIKE YOU, youy will never get better.