The advantage Nadal has over Federer in pursuing a Career Grand Slam is that Federer can't beat Nadal on clay and the gap is getting wider, and people would be shocked if Federer beat Nadal at Roland Garros after we just saw Nadal beat Federer in the most lopsided Grand Slam Final of all-time.
Whereas if Nadal beat Federer at the US Open/Australian Open it wouldn't shock anyone, because Nadal matches up very well with Federer.
At first people said Nadal only owns Federer on clay, but then Nadal matched him at Wimbledon in 2007 and then beat him at Wimbledon in 2008. So nobody is going to say Nadal can't beat Federer on hardcourts, especially after Nadal outplayed everyone on US hardcourts in the pre-US Open tournaments/Olympics and beat Djokovic when Djokovic was putting his complete effort into winning a medal on a freakishly fast Olympic hardcourt.
To add to that, Federer is looking less impressive every year as he gets older, while Nadal is improving his results every year at ALL the grand slams.
You can makes some desperate prediction about Nadal's body crumbling, but bare in mind that Nadal is no Lleyton Hewitt, no Michael Chang and no Jim Courier. There has never been an athlete like Nadal in tennis, and we are still learning about his capabilities much like we learnt about Michael Jordan as he broke record after record by winning the MVP at age 35, retiring and scoring 51 and 45 in back-to-back games at age 39 and at age 40 was the only player to play all 82 games in his team.
These are unprecedented, and Nadal is patterning his history after unlikely scenarios such as being the first player since 1980 to win the French Open and Wimbledon in the same year, something he could repeat each year in future given the decline of any credible grasscourt opponent and his comfortable dominance at Roland Garros.
