Let's get the obligatory disclaimer out of the way: Roger Federer's #1 ranking is not on the line. It won't be for months. He isn't really playing for rankings here; he's playing for the record books.
The #2 ranking, though, is very much in play. Andy Roddick still has a small lead on Lleyton Hewitt in the rankings, but Roddick has more to defend. We can treat them as effectively tied; unless they both lose very early, whoever lasts longer will be ranked #2.