Top half of the draw featuring all top seeds in QF, whereas in the bottom half only Federer left.
In fact, the players from the top half of the draw currently hold top 4 places in my implementation of Elo ranking system:
Alternative player ranking system
(I only post weekly rankings there, but current situation after R16 matches completed is
1 Novak Djokovic 346.6
2 Rafael Nadal 326.2
3 David Ferrer 275.2
4 Tomas Berdych 269.6