It's incredibly rare for a underdog to take out a top ranked player in Tennis. However, it can happen but the odds I have seen so far make sense.
It's not incredibly rare, tennis is always famous for underdog beat best players.
Perhaps incredibly rare was not the right choice of words. When players aren't injured, it's uncommon. Nishikori skipped Cincinnati Masters and he was the only upset as far as I know.
Safarova also upset and some very low ranking players beat good ranking players.