I think Sampras' "endurance" problems are a bit overrated personally. It's not like he was keeling over in his matches routinelly. The man played a TON of matches as all pro tennis players do, and I've only really seen him in that "keeling" mood a few times. In the Corretja match, the reason he got all weasy in the end was not because he didn't have endurance, it was because he was drinking coke on changeovers because he felt a little down. Had he not drunk the coke, I'm sure he would have "survived" ok. The Yzaga match? The guy was sick, it happens. Guys gets sick now and then, it doesn't mean he was an endurance cripple or anything. By and large, I always thought he held up fine physically in long matches, this in spite of not training for endurance at anywhere near the same clip as Thomas Muster and his 10 miles a day did. Granted a lot of this has to do with his serve keeping things short, but still the reality is that Sampras very RARELY was all keeled over and stricken during his matches.
As for Sampras vs. Federer. I think Federer primarily has problems with two things. Guys who can mix-in net approaches against him, and two guys who can hit high bouncing topspin. The high bouncing topspin causes Federer to make more errors than normal as the balls are not in his ideal strike zone, and I do not think it favors either his topspin backhand or his MUCH relied upon slice backhand (no other top player since Edberg has made this as much an integral part of his arsenal).
Two, opportunistic baseliners who know when to sneak into the net. Federer floats back A LOT of slice backhands as change of pace plays and whenever he's in trouble. He's so fast from the baseline, that when he's in trouble, he's not really in trouble. He just floats a slice back, and the point is basically reset...at which point you have to like his chances. I feel like Kiefer and Santoro give him competitive matches because they're very opportunistic. Yes, they're baseliners, but they're also always thinking of ways to sneak in and knock off the easy volley. Federer DOES give you these opportunities, but it's a bit like playing Capture the Flag. You have be sneaky about it. If you're all obvious about it and come charging for the flag, Fed will bop you on the nose. The few times Agassi snuck in against Federer he won the point easily, but the problem is he's not confident in his volley abilities and his volley can easily produce a lot of "flubs" as well, even on sitters.
Once Fed catches on to this ploy, he's dangerous but not invincible. Good, solid volleyers like Santoro and Kiefer will get their share and accomplish the goal of keeping Federer guessing for once, keep him honest if you will.
Finally, Fed used to have trouble with consistent guys as well. Agassi used to own Federer, because this was the "modern" Agassi, uber-consistent and the ultimate percentage man. Nalbandian and Hewitt used to own him for the same reason. The problem all three guys have now is that, Federer is no longer spooked by consistency. With Nadal it's different, because Nadal's consistent but it's consistency that is caused by high bouncing topspin. Nalbandian, Hewitt, and Agassi all hit flatter trajectory balls. This is right in Federer's strike zone and gives him I think a certain comfort zone that he doesn't have with Nadal. Nadal doesn't give Fed problems because he's consistent, he gives him problems because Fed like all one handers in the history of tennis except Muster and Kuerten does not like high bouncing topspin balls to his backhand. It affects both his slice and regular backhand strokes, and also creates a problem where he begins to second guess himself and hesitate on the execution of his inside-out forehand. I feel that Federer is a bit stubborn. If he starts making errors with a certain shot, he tries to hit it many more times to prove it's not a weakness. I feel this is his own arrogance baiting him to "correct" whatever might be going off in a match.
Bruguera beat Fed 6-1, 6-1 coming right off major shoulder surgery that doctors said was for sure going to end his career. Granted, Fed was not the same player then, but he also was already showing his promise that year. I have this match on tape, and Bruguera played as abysmally as I've ever seen him. He looked like an absolute shell of his former self. He was slower than molassass, repeatedly late on almost all of his shots, slow to react and anticipate, no confidence in his strokes. Just really awful. Yet, Fed was even worse this match. Fed missed routine balls over and over, he made numerous errors with his backhand, and just all in all looked out of his comfort zone.
To me, it's a combination of both. Fed back then had problems with consistent players. Now Fed no longer has problems with consistent players UNLESS they have the potential to hit high bouncing topspin...ahem, Nadal.
Finally, Hrbaty? Ehh, shiznit happens. Don't really get that one, but I guess everyone has their boogeyman.
Anyway, I think Sampras had the better serve for sure. Say what you will about Fed's "numbers," but all I know is that whenever Sampras got down 0-40 in a game, you almost expected three consecutive aces or unreturnables to bail him out which would just demoralize his opponents. Meanwhile, Fed when down in a game, takes on a different mentality. You can see it in his eyes, he takes on the look of Michael Chang. That mentality, that alertness, that I'm going to run down every and ANY thing you're going to throw at me during this point. Federer has those "counterpuncher" instincts in him that Sampras didn't. The difference between Fed and Chang is that Fed will outright end a point at the *slightest* opportunity. He has the speed to retrieve until hopefully you miss out of neverousness, but if you don't? No problem, he'll just rip a winner OR...VERY often...bait you into the net with a fast dying slice, then when you pop the ball up, he makes the easy pass.
Fed is more likely to "construct" his way out of jams and make it interesting, whereas Sampras simply just ends your hopes with a THUD. Fed usually does too, it's just that it might take a few shots into an ensuing rally before he does. Fed is the kung-fu master, Jackie Chan with the smirk. Sampras is something out of an old Western, the gun shot right between the eyes.
Finally, on the volley. No question Sampras is better. Fed volleys when he makes a great opening for himself. Take away Sampras' forehand, and have him serve and volley full-time and he'd still be a legtimate top ten player. I can't say the same for Federer. There aren't as many GREAT volleyers anymore so Fed looks great by comparison. But back in the day, there were guys like Scott Draper, Wally Masur, Cedric Pioline, Jason Stoltenberg, Todd Martin, Mark Woodforde, Todd Woodbrige, etc. who could volley exceptionally well without being "well known." These days if Mardy Fish is considered a great volleyer relative to his peers, something is wrong.
Remember, Federer says the reason he does better at Wimbledon now is because he ditched the gung-ho, serve and volley approach. Fed actually now stays back much more at Wimbledon, and picks and chooses when to approach. He was formidable serving and volleying only, but invincible or even top ten caliber? Hardly.
Overall, Fed is the more whimsical player who often times is content to simply start a point by chipping a serve back into play, etc. He can do this, because he's so confident in his speed and court coverage. He's willing to both play the "pusher" and the "fencer." He'll gladly give you the opportunity to miss and donate him points. If you don't, he'll gladly jab you and scrape you and otherwise massacre you until you look like modern art confetti.
