Actually, in order to advance to round 2 you have to be top 1000 in one of the first rounds, so you make it to the top 3000.
The reason I asked is because I placed in the top 1000 (top 1000 of round 2) back in 2011 but was only contacted by a google recruiter in late 2012 (in the 2012 edition I had been just outside of the top 1000 in round 2).
There is qualification round, round 1a/1b/1c, round 2, round 3, onsite finals.
Advancing to round 1 won't impress anyone because qualification round is easy.
In order to advance to round 2 you have to be top 1000 which is pretty difficult but achievable.
Advancing to round 3 is very difficult, I think it's way beyond interview level.