RE: Rafael Nadal - From Peak to Plateau
As Moxie pointed out, I'm differentiating "Peak" and "Plateau" and also "Plateau" and "Decline." Peak is a level of performance consistently above career averages, especially with regards to winning percentage, while Plateau is still very good but closer to career averages, and Decline is a sharp fall.
So, for instance, Roger's plateau range also included one year--2012--that was solidly above his career average (86% vs. 81% career), but not nearly as high as his 2004-2007 peak (88-95%). It looked like he was entering Decline in 2013, but he has since righted himself and seems to be hanging out in Plateau for a bit longer.
It is also different by player. Roger's best three years by winning percentage--2004-2006--are considerably better (93, 95, 95) than the rest of his career (81), while Rafa's best years, 2008/10/13 (88, 88, 91) don't stand out as sharply (82).
But the point being, what I am calling Plateau is not steep or even clear Decline. It is a stabilization of a high level of play "post-peak" but not as consistently good as a player's performance at their best.
As an aside, one could argue that Djokovic had only one peak year (2011, 92%) but I think there's a difference between "ultimate peak" - which could be a player's very best year - and "peak phase," which is a period of at least a few years. Novak is still in his peak phase, as evidenced by his winning percentages over the last few years: 92, 86, 89, 86 vs. 80.6% career.