Well this thread is about all of Federer, Nadal, Serena, and Henin, and to determine where the bigger gap exists on must consider not only Federer and Nadal but Henin and Serena.
Henin making the 2001 Wimbledon final by beating Capriati and taking Venus to 3 sets marked the start of her prime IMO. She would have won the 2001 French if she didnt choke a big lead in the semis to Clijsters, Capriati was never beating her there. Had that happened nobody would doubt 2001 was the start of her prime. So mid 2001 marked the start of her prime (not peak, but prime) for me. 2002 she was co-favorite for the French title along with Serena until her injury at Roland Garros.
Even if you disagree 2003 was one of her peak years and not only did Serena nearly beat Henin at the French (probably would have without Henin's desperation and near defeat pulling a famed cheating act) but the highly respected Mary Joe Fernandez declared pre match that Serena's best on clay is better than Henin's. Of course the expert validated fact that Serena's best on clay is better than Henin's does not mean Serena is the better clay courter, Henin is far more consistently close to her best on clay than Serena is to hers. It does indicate that she is closer to Henin on clay than Federer to Nadal though, as nobody would ever say before a big match in one of Nadal's peak years (or any year Nadal is playing tennis for that matter) that Federer's best is better than Nadal's best on clay.
Dude, I was just correcting someone by saying Federer never took Nadal to 5 at the FO but he did at Rome so maybe that's what he's thinking of.... which he was, as he said himself. I don't have to discuss the Serena vs Henin factor when I'm correcting someone on Federer Nadal...
Anyway I don't think Federer beating Sampras at Wimbledon marked the start of his prime, i don't think nadal beating Federer at Miami 2004 marked the start of his prime. But you could be right that 2002 was her prime, but I'd say the beginning, not 2001
Also who made Mary Joe the final word on everything? Her's is an opinion, why are you so hung up on it? I understand what you mean (what she means) totally, but she can't be proved right.
Also you could argue that Federer's best is the first set of RG 2006 where he didn't allow Nadal to settle. Even on clay nadal often needs a few good games to get comfortable, so in the highly unlikely event that Federer just played liked that and hit lots of winners to very few UE's then his best could beat Nadal by not allowing Nadal to even play. It's got about 1% chance of happening, but as a more attacking player he always has the chance to dominate on any surface for a while - of course it's not usually for more than one set on clay.