I'm amused by this discussion, guys. A textbook example of
belief bias, i.e. you see what you want to see rather than what is shown. The funny aspect is that there is no proof for the reality of either views because each one of yous will claim their version based on different video recordings where clock going down to zero shown or not shown or shown at different angle, etc.
I must chip in to this discussion here, and say how stupid organisers have been to feed such discussions by
stopping the clock at zero. Either they should bang the siren when the clock reaches zero (like in football), or
do not stop the clock but let it count down minus zero! And the clock should be reset only after the point is over (and starts counting for the next point), i.e. it'll remind everyone that the server was late at the beginning of the point That way, everybody can see the reality (without bias), and with each passing minus zero second, the crowd is incleasngly likely to boo Nadal (if siren did not boo him at zero) and the umpire will face increasing pressure to explain his abuse of "discretionary power" to overrule the rules. Further, snappy reporters will be zooming at Nadal's serving motion with a big number "-5" behind him, and sending the pictures to all tabloids around the world. I guarantee everyone, especially Nadal, will quickly learn how to "beat the clock", just like football players did. Let me remind you few years back how players used to look at their boxes before challenging. Nadal was looking at Uncle Toni every time. Somehow they were taught said look was illegal by consequent umpires, and no one does it anymore. Time to teach players how to appreciate the clock. The technology to do so is there: I repeat either boo the siren at zero, or don't stop the clock and let people and reporters boo the sticky minus zero number.
EDIT: I forgot to mention when the clock should actually be stopped if not at zero. Very simple: the hawk eye system detects the service motion/ball in motion and stops the clock. As I said the technology exists to do it right, only organisers' willingness to treat the clock seriously is missing.