It's not just Nadal's knees that must stay healthy. His body, his focus, his mind and his determination must be absolutely spot on for the next 4-5 years.
He must be able to play at his prime level past his 30s which historically has never been done in the history of this sport.
The question is, how many years does he have where his body can still handle the ferocious grinding 4-5 hour battles against younger players?
Every year, things get tougher and tougher both physically and mentally for players who get up in age.
Just about everything must go right for Nadal for many more years in order for him to threaten such a record.