The crucial factor is player skill and customization.
In the case of two 1st level fighters, we say 50-50, because we reason that someone has to win. But let's say you get a min-max'd human fighter with optimized feat selection for fighting other fighters, and the second fighter is an elf fighter with say.. "Iron Will" as his feat.
I'd say that elf is going down, and it's going to be a lot more likely than 50%. Even if the elf is second level, he's probably going down. At third, depending on how good the player behind the 1st level guy is? That elf may still be going down.