On losing the will to live, that's what I meant by burnout. I would say the main characters on the men's side who stopped winning because of it are Borg, McEnroe, and Wilander. As you point out, Agassi recovered, so he doesn't count as more than temporary burnout. Djokovic might burnout, but it seems unlikely. My speculation would be that burnout is more likely to happen to players from a relatively privileged background, as they are more likely to start questioning the meaning of life, something that less privileged people rarely do to the same extent. Djokovic grew up in a war zone, so I think he won't burn out. But this is just speculation. I can't prove it and have no intention of trying.
Agreed that recovery times are improved because of treatment and surgery. I was reading Becker's autobiography last night, and he says the same thing: that treatment has improved out of all proportion since 1985. This on its own ought to make it easier for players in the late 20s and early 30s to keep competing despite the increasing build up of niggling injuries.
About Lendl: his #1 position in 1989 is hotly debated, as Becker beat him at both Wimbledon and the US Open and claimed both titles. Even if you take Lendl to be #1 in 1989, he was clearly less dominant than he had been in 1986 and 1987. In both those years, the only major matches he lost were on grass (Wimbledon final in 86 and 87, Australian semis in 87), as he won Roland Garros, the US Open, and the Masters (YEC) in both years. But in 89, he was a narrow #1, not a dominant #1. Besides the Becker defeats, which are partially explainable by the head to head advantage Becker had over him in major matches, he also lost to Edberg in the semis of the Masters and, inexplicably, to a cramping 17-year-old Chang in Roland Garros from two sets up. So, I'd say that his decline was gradual. Also, the WTF voted him world player of the year in 1990. All in all, I would say that his performance in 1990 wasn't that much worse than it had been in 1989, and that his performance in 1991 wasn't that bad either. He didn't win a Slam, but he still beat the #1 in the Australian Open semis to make the final (and lose to his nemesis), and beat the Wimbledon champion to make the US Open semis. His decline definitely accelerated in 92, but even then he finally beat Becker in a Slam to make the US Open quarters, and but for rain would probably have beaten Edberg from two sets down in the quarters. He'd have had a shot at beating Chang in the semis too, but probably not Sampras in the final. Only in 93 did Lendl's decline become terrible.
Wilander did not come of age in 1990! He was finished by then. He came of age in 1983, I would say. Some experts thought he deserved to be the #2 that year, even though he was only 18 until August.
Federer's Slam performance was indeed great in 2009, but his performance in other tournaments was already far below what it had been in 2004-6, and even in Slams he was being pushed harder than before. I think that in the early stages, decline is mostly marked by declining performance in minor tournaments, as the player has to really reach overdrive to keep winning, and by being pushed more in majors.
Decline can be sneaky and gradual, or it can happy suddenly, and in the past it mostly happened faster because surgery was not as good, there were not as many legal "treatments" for all sorts of things, and players get physical help now that did not even exist.
In addition there are players lose their will to live and the discipline to train, true of Borg, JMac and Agassi. Agassi is probably the only player I can think of who made a complete turn-around.
Lendl was pretty much on top through 1989 and started on that path in 1985, I think. He fell to #2 in 88. But starting in 1990 suddenly a whole bunch of very talented people came of age - Wilander, Edberg and Becker and were born between 1964 and 1967. Those three had 19 slams between them. It's a lot like what happened to Fed starting in 2008. A lot of Nadal's slams came earlier, on clay, but if you count up the slams won by Novak starting around that time, add in Nadal's later FOs and other slams, it's a little similar. Only Fed did not have to work his way out of anyone else's shadow.
And this is nothing against Fed. It's just the ebb and flow of the way things change. Every now and then history is ripe for a new champion. It was a lot like that for Connors, who did not run into major competition until later.
Most people overlook the fact that between the USO of 2008 and AO of 2010 Fed was in every final and won 3 out of 5 in a row. 2008 was an anomaly that can be explained by sickness. Through the AO of 2010 no one had reason to believe that he would not continue. The change after the AO of 2010 was like falling off a cliff.
Nadal's decline has been more rapid, but he had his injury problems for many years, and his form in the first half of 2014 was already shaky.
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