I don't know if anybody else saw this, but during the Wimbledon studio coverage, Darren Cahill and Brad Gilbert made the point that both Serena and Graf were/are great athletes (as compared to somebody like Sharapova, who's just really good at tennis).
They told a story about how Graf used to train by running 400m intervals on a track. The German Olympic track coach happened to see her once, and timed her... and her times could have put her on Germany's Olympic relay team.
It's really hard to compare different generations in tennis, because the racket technology keeps advancing, and the surfaces have become so similar. I think tennis is clearly being played at its highest level ever right now, but that doesn't mean that past players, given access to today's equipment, couldn't have competed with today's players.