What are the chances of him getting to #1 in the USO in case he loses on Friday?
if Fed loses the final on sunday, the chances would still be high, and even before the US Open.
here are the ranking points until Shanghai when you take off the points from last year :
after Wimbledon :
(please note that the Olympics give 750 points but they are an optional tournament, which means Fed could only replace his 90 points from Doha with a 750 points win, then it could not be enough to be number 1 after the Olympics)
after Canada open (won by Djokovic last year then he would drop 1000 points) :
... which means Fed would be number 1 if he doesn't do worse than Djokovic in the Olympics+Canada open
after Cinci (Djokovic was finalist and will lose 600 points ... which is even more favourable to Fed) :
after the US Open (Djokovic will have at least 1885 points to take back to Fed until then !!) :
after Shanghai :
By the way, here's the Race (situation in the end of the year) :
Djokovic AND Nadal 6840
(Murray 3800 if you want to know)
... which means Fed is in competition for the year-end number 1 !