SS doesnt need to be able to physically react to zoom. All he would need to do is perceive him and he would be able to throw up a forcefield(which he does with a thought). While the flashes senses are sped up, they r still regular human senses. SS senses are sped up and advanced far beyond regular human senses(both traditional senses and cosmic senses). His cosmic sense will instantly alert him to the presence of Zoom even if he cant see him with his traditional senses. SS could then create a forcefield (which he does with a thought) which would pretty much be the end of Zoom.
http://img90.imageshack.us/img90/3006/silversurfer199612206jx3.jpg
Here SS races past billions and billions of stars and is able to focus upon each of them. Though each star appears to him as only a spark of light , he is able to note the position of each and calculate his position in the universe. This feat is relevant because it shows how fast SS traditional perception is and how far greater the speed of SS thought process is of his already uber fast perceptions. SS is able to focus on each of the billions and billions of stars he passes, though they appear only as sparks of light(which is a testament to the speed he is moving at), the fact that he is able to visually perceive and focus on each and every spark is extremely impressive. However it is even more impressive that even after only seeing sparks of light, he is able to mentally note each of their positions and simultaneously use them to calculate his position in the universe. His mental processing and reaction speed and is therefore much greater than that of his visual perception.
Hence if SS is even able to visually perceive zoom slightly (whether as blur,flash of yellow,etc), his mental speed will enable him create a forcefield in time to defend against zooms attack. Since SS perceptions are highly enhanced as well as sped up (unlike flash whose perceptions are only sped up ), visually perceiving zoom even for the slightest of moments is certainly a possibility. That however is not even of great importance considering that SS non-traditional senses i.e his cosmic sense will immediately alert him to zooms presence. Hence even if he cant see zoom in the slightest he would still be able to erect a forcefield (which is done mentally) as soon as he is aware of zooms presence via his cosmic senses (which will be immediately).