Rafa's 2 best slams since 2008 were 2010 USO and 2013 USO.
His game has gradually been trending toward hardcourts (especially hardcourt slams).
Even indoor hardcourts, he's made the Final of the WTF twice, 2010 and 2013, not early in his career.
Rafa has won 4 of his last 5 slam meetings with Djokovic, the most lopsided was 2013 USO (6-1 in 4th set).
I tell you the guy that should be worried most about the North American Summer is Djokovic (titleless in 2013 and 2014), not Rafa.
Rafa loves fast hardcourts, it makes his serve and groundstrokes more dangerous, while it gives Djokovic less time to use gymnastics.