Since the 2005 French Open, the only grand slam that was not won by Roger Federer, Rafael Nadal, or Novak Djokovic was the 2009 U.S. Open won by Juan Martin Del Potro.
Adding in Andy Murray, no one else made a final last year (Murray and Federer 1 appearance each, with the other 6 slots split evenly by Djokovic and Nadal). Those four are the only men left at this years Australian Open, which means 4 of the last 5 Grand Slams semi finals comprised those four (last year at Wimbledon Federer lost in the quarters to Tsonga or it would have been 5 straight).
I don't ever remember a time where 4 men (I'm including Murray since he is clearly way better than everyone else) dominated the rest of the field like this. Totally impressive, especially since they are all doing it on all 3 surfaces.
The questions I'm struggling with are
Is this good for tennis? If not, what can tennis do about it?
What does this mean for American tennis (since none are American)?
Who is going to be the person to challenge those four?