Its a risk you take by readying an action.
Personally, if I was going to take this readied action I would do.
"If my oponent takes a 5ft step I will step and trip him."
This means that your opponent has take a 5ft step and if your lucky spends the rest of his round getting back up.
Personally, if I was going to take this readied action I would do.
"If my oponent takes a 5ft step I will step and trip him."
This means that your opponent has take a 5ft step and if your lucky spends the rest of his round getting back up.