Well, as I said, it doesn't matter when you compare Federer to Nadal, or Nadal to Djokovic, because all them benefited greatly of the homogeneization of conditions, of the unique baseline game, of the 32 seeds at GS tournaments, etc
But of course it is unfair to top players from other eras where they hadn't 32 seeds at GS tournaments, they hadn't the luxury of being able to play the same style all year long and still win because anyone else also plays this style all year long and courts allow it, etc
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Tennis is a zero sum game for all the competitors.
If the surfaces play more similarly, there are more potential competitors at every slam. The contending pool increases, which makes it tougher to win slams when you dont have specialists who thrive on extreme conditions.
On grass in the 90s, the contenders were VERY different to the contenders on clay.
Today, everyone is a factor everywhere. Players like berdych, tsonga, del potro find it close to impossible to break through the first tier of players - fed, nadal, djok, murr because they are factors at every slam and the second tier of players has to beat them in succession.
If you are in the third tier, forget about it... no more malivai washingtons, piolines, voltchkovs that took advantage of smaller pool of competitors on grass in the 90s.
You are going to have beat del potro/tsonga/ferrer/soderling and then the first tier from the qf onwards.
It is not only me. Many players (Berdych, Tipsarevic, Federer....) have said conditions today (meaning mainly speed) are more or less the same everywhere (except for the differences in movement on grass, clay and hard).
Federer has said more than once that it was totally different in the 90s, and the fact that today all the courts+balls are similar and slower plus 32 seeds and the end of surface specialists, makes it way easier for current top players to always make the finals rounds of all GS tournaments, that in the 90s a red-hot hard hitter in a godly day (and there were many of those) could defeat anyone on a fast court, but today that is almost impossible given the conditions.
So at least Federer is honest and brave enough to acknowledge all this even if it means he (and other top players from current era) wouldn't probably have those achievements/streaks in previous eras conditions.
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off the top of my head...
Nadal has been beaten by gonzales, tsonga, del potro, soderling, ferrer, youzhny. He also survived scares from petschner, haase, youzhny, kendrick on grass. Those matches could have gone either way...
Djokovic was beaten by kohlscheriber, tsonga, berdych. roddick..survived tipsarevic, seppi, wawrinka..
Give me an example of a player in the 90s who had a red-hot day and took out a player of comparable calibre to the top 4 today (i.e. a flash in the pan player?)
Sampras was beaten by krajicek, but he is of the calibre of tsonga, del potro etc. Im sure good players have beaten by other good players. But not many shocking upsets of someone of the calibre of djokvoic, nadal, federer..
Of course in the 90s, the only player i consider of that calibre is Pete Sampras.
Also, keep in mind that because the competition is intense throughout the calendar year - nadal, djok, fed, murray are all contesting finals in every other tournament on a regular basis - they are all burned by the end of the year. Their careers are likely to be much more physically challending than their predecessors, which makes longevity much more difficult.
i wouldnt call it a "luxury".