Roddick Wimbledon 2004 Forehand winner : 104 mph
Gonzalez Wimbledon 2005 Forehand winner: 106 mph
Safin Roland Garros 2004 Backhand winner: 102 mph
I know Monfils has hit harder than this but I am not sure of the exact speed. There is no way a backhand or Forehand can be something like 120-130 mph therefore for the Blake forehand it's probably the speed of the serve he returned. Same thing with Sharapova supposedly hitting a 120 mph Forehand Winner when 120 mph was the speed of Serena's serve.
That is what I thought as well. I was thinking anything around 100 is pretty fast. When Fed hits 80-ish MPH winners they are pretty crushing blows.
When you think about it, have you ever seen a groundstroke that you really thought was as fast or faster than a serve? Given the nature of the strokes, it hardly seems possible.
I suppose if you measured the speed of overhead smashes at the net, you would get some pretty fast strokes. But, that is not what the poster is asking.