Originally Posted by pc1
While the OP has made some excellent points, there are several things to consider when we look at Sampras' percentage of service games won numbers versus the few who are ahead of him. First of all we have to consider that in his time that Sampras usually led the ATP in percentage of holding serve. This could mean several things. One thing is that considering the equipments, surfaces and overall conditions it may have been harder to hold serve in those days and a slightly lower number may be good enough to led the ATP. Another thing is that Sampras went deeper into tournaments than Isner, Roddick or Karlovic and faced better returners like an Agassi, Chang etc. The last thing is perhaps the others are just plain better. Personally I think guys like Isner and Roddick are fantastic servers but to me it's debatable whether they are superior to Sampras. Roddick's numbers in leading the ATP however are fabulous and I do think he is on Sampras' level, maybe above or just below him.
Most people agree that current conditions favor returners much more than servers compared to the 90's. Slower surfaces and better strings mean that it is easier to get the ball into the court with interest. Overall I think Isner, Roddick, and Karlovic as well as a few other players (including from Sampras' own era) have better serves than him. None of them, of course, could match his overall game or mental strength which is why they achieved so much less.