Snoopy: It depends on what kind of match type is used. If it's a single game (or another fixed number) then as others wrote, the one with the lower ranking goes through. But if the match type is one win, in case of a draw, there's a rematch.

It took me a while to figure this out. I don't really like the idea of a player going through based on her rating; therefore when I create an elimination tournament, I try to remember selecting "1 win" as the match type.