Here is an interview with Richard Krajicek in 2004.
What do you think of Roger Federer, the Swiss world number one?
If you are only looking at the tennis and what he has done last year, you will think that he is better than Pete. Which, maybe, he is. But Pete had the desire to play at the highest level for so many years. That is very difficult, mentally. That in turn is the biggest question for Federer. Can he maintain the high level over six-seven years to break the record of 14 Grand Slams? Will he be able to remain world number one for five-six years in a row?
That is actually the only thing he is yet to prove. But if he continues to play tennis like this, he will definitely break Pete's record. Federer can be the best ever; he has the potential. The only question is whether he has the desire.
And even earlier in 2003, right after Federer won Wimbledon for the first time.