That's a good point. Stats don't tell the whole story. Perhaps it's a combination of both.
If we look at ROS merely as a stroke, we would probably need to divide this into 2 categories -- Fh ROS and Bh ROS. After all, the ROS is a special case of the larger category, groundstrokes.
But the ROS may be considered more than just a stroke. Those who excel at returning serve are able to read and react to serves better than most. They are able to handle challenging serves and, in many cases, neutralize them.
It could be argued that Novak has a more aggressive/offensive ROS, in general, than Rafa. However, they are nearly equally effective. Perhaps, Rafa a bit more so than Novak. They are both able to hit returns, a good percentage of the time, that are not weak replies that the server can easily put away. By hitting effective returns, they can often offset the advantage of the serve and stay in the point.
The bottom line is that both Rafa and Novak are very effective returners of serve.
yes good points. i basically agree with everything you've said, but i guess i'd like to elaborate on my comment i made about the ROS as a stroke vs return games won stat and why the stats regarding return games won doesn't really reflect effectiveness of the return as a stroke. also elaborate on why i feel djokovic is a better returner than rafa. i'm not saying you disagree, but in any case, i think i've arrived at some solid conclusions i want to share.
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i guess i view the return of serve as a stroke that can be used to get free points, set up free points (a 1,2 punch), or force an error without making an error of your own. much like how the server utilizes his serve to win a point or set himself up for a forehand winner/volley, the returner utilizes the return to do the same thing.
rafa is very good at neutralizing serves. it's very good at helping him set up his ground game, but after his return is returned by the server, unless he can put his opponent's reply to his return away for a winner, his return is no longer affecting the out come of the rally since it's now all up to his groundgame. this is especially true if rafa hit's a neutralizing return since the return is neither aggressive nor poor, just a stroke to make the rally become immediately neutral. as far as neutralizing the serve using the return of serve(as a single stroke), i would say he and djokovic are just about on par right now. both are able to get to just about any serve and immediately neutralize the rally fairly well. it's hard not to mention federer, since for most of his career he's been slicing back big serves with great placement as well, but he was scarcely aggressive, even with his forehand in his prime. but of those three, djokovic is the better returner since he's able to be so good defensively but can often hit winners or force errors with his return.
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i found a list showcasing the 10 players in the last 20 years with the greatest service return games won percentages. although it was published on bleacher report, the stats seemed solid since they were actual, objective stats.
# player name /%of ROS games won/ matches
1. Guillermo Coria /35/ 332
2. Rafael Nadal /33/ 652
3. Alberto Berasategui /32/ 477
4. Filippo Volandri /32/ 333
5. David Ferrer /32/ 631
6. Andy Murray /32/ 426
7. Jordi Arrese /32/ 246
8. Andre Agassi /32/ 899
9. Thomas Muster /32/ 625
10. Michael Chang /32/ 810
http://bleacherreport.com/articles/911819-the-best-returners-of-the-last-twenty-years
it lists coria as number 1 winning 35 % return games, nadal at no.2 winning 33% and 8 others at 32.xx% including murray and aggassi who are both good returners. all of the players on that list are great if not world class defensive baseline players. perhaps agassi is more of an aggressive baseliner than the rest and aside from murray, is the only one to find most of his success on hardcourts. however, of that list, i've only heard of agassi and murray being heralded as having great returns, the rest are just decent/good returners with stellar defensive games. some other interesting numbers i've found on the atp site:
djokovic with 31% career return games won
connors 17% (granted connors played with wood+gut)
federer 27%
hewitt 21%
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it's always hard to compare players who didn't play in the same era since there is no constant control to relate to. that said, if we are talking about making something happen just by purely returning the opponent's serve, not returning groundstrokes in a rally that's already 4+ shots long, i'd have to second your notion of agassi as having the one of the greatest returns(in my eyes the greatest), since he is able to hang with the defensive guys who have great return games won percentages, but can also anticipate serves and crack returns like no other.
unlike a number of others on the bleacher report list, he doesn't wait for serves 10 feet behind the baseline, he is able to react while practically on the baseline. with that in mind, that's where i think nadal falls short to his great-returner-contempories djokovic and murray. when recieving, nadal returns from a much farther distance away from the baseline compared to djokovic and murray(murray starts from behind the baseline but moves up to make contact close to the baseline). as you said those who excel at returning serve are able to read and react to serves better than most. again, i agree with most everything you've said, this is essentially just some conclusions i came to upon digging up some more info.