This again shows how clutch Nadal is, having such a huge percentage (in his career it's pretty high as well I believe) without a big serve that can help him out.
Nadal has always had good stats in his service games and not that good stats in his returning games, which just shows that his serve was not bad at all.
Comparing to a Murray or Djokovic (rather return-oriented players), he has always been a more serve-oriented player.
And his serve has improved in recent years, I've seen many sets in recent years where he just stayed there thanks to his serve.
And actually his tiebreak stats were not that good in the past, they have improved in recent years coincidentally with his serve improvements.