Bursztyn: When I check a player's early-round loss total, I look to see whether there has been a dramatic increase (or decrease if it's in the early part of his career) in the number of such losses. It's usually a good indicator that perhaps his best days are behind him. It has nothing to do with comparing eras. I'm sure you'd find it holds true regardless of era. As an example, I'll look at Goran Ivanisevic's numbers, as the 2nd part of this post will be about him anyway.
1989 14 of 23 tournaments ended in early losses for him
1990 11 of 24
1991 11 of 24
1992 7 of 23
1993 9 of 23
1994 8 of 26
1995 10 of 24 one of them due to retirement with injury
1996 8 of 29
1997 10 of 24 one of them due to retirement
1998 16 of 29
1998 was the year that GI regressed significantly. His W-L record also took a hit.
RF 2004-06: Your assessment of GI's physical skills in no way comes close to matching the reality, and I'm talking about what ultimately counts in sports--the wins and losses columns. If GI actually had the things you credited him with, he surely would have done better than to finish his career with a 599-333 record, to go with 22 titles. Of those 22 victories, 15 came indoors. I trust you can draw the proper conclusion. In addition, his outdoor hardcourt winning % is a meager 58.2, one of the worst I could find for a Top 10 player. Finally, his clay-court % is 61.8. Even the much-maligned Andy Roddick did better, at 63.6. That mental toughness must have been absent far too often.