But what would the starting points be? Imagine you're at Doha 2013. Year just started. You're calculating the "race" ranking. Federer wins 1st round against....say Tomic. What is their ranking before and after this one match? Something like 1500 --> 1520 for Federer and 1500 --> 1480 for Tomic?
Well, the floor in Elo rating system is actually arbitrary, and most commonly it is set to 100 or 400. So, both Federer and Tomic have the same, say 400 points, to start with. In that respect it is not very different from ATP race list, where everybody starts with 0 points. So, if Federer beats Tomic, he would then have 412 points, and Tomic would fall to 388 points. But, with ATP point system it is not the same if Federer beat Tomic in final or in R32 match, whereas in Elo system it is not important in which phase of the tournament a player wins.
Interestingly, if the floor was set to 2000 instead to 400, the difference between the 2 players after the match would stay the same +12 points, and -12 points - 2012 and 1988, respectively.