When both are on their games, neither can be beaten. In the Federer-Safin match you are referring to (2005 Australian Open I believe), Federer was off his game and Safin played the match of his life and he still barely won. Rafael Nadal can beat Federer routinely on clay but there have been clay court specialists for years in tennis (Gustavo Kuerton a few years ago). As far as somebody challenging Tiger, in the last 3 years (12 majors 04-06), Tiger has won 4 majors. Second to him? Phil Mickelson with 3. In tennis the last 3 years (13 majors 04-06 and this years Aussie Open), Federer has won 9. Second to him is Nadal with 2. Federer has made the final in his last 7 majors (tied for the most all time). He went 92-5 last year and only lost to 2 players (Nadal 4 times and Andy Murray once). He's the first player in the Open Era to win three of the four majors in a year twice. Don't get me wrong, Tiger is very good and very dominant, but the answer is Federer and it's not close.