IMO Federer can be subject to mental problems, but against very specific adversaries and in specific match configurations. In general I wouldn't say he's bad mentally speaking, but Nadal and Djokovic definitely managed to exploit a crack in the armor. It remains unknown if Sampras would have been able to do the same.
I'm a big Agassi enthusiast too, but I wouldn't say he was the clutchest player either (as in he was good mentally, but not the best). He had definitely some chances against Fed that he didn't manage to take, for instance in the crazy 2004 USO QF.
Yeah the wind in that 04 U.S. Open quarter was unreal, that's why I was hoping Agassi could get him the next year.
Playing 5 sets against Ginepri on Super Saturday the day before taking on peak Fed probably hurt his chances as well.
Though physically, Agassi looked fine against Fed in the final.
Agassi's clutchness is interesting, Pete seemed to be the one that could get in his head a little, but I think that came down to Agassi feeling like he might get one good look a set at a break on Sampras's serve, thus feeling the pressure of those few BP opportunities.
Agassi also had some crazy losses, Courier at the French early in his career when he was ahead, then the stoppage, then losing (with the coaching Courier got in between the stoppage).
Agassi's loss against Rafter in the 01 Wimbledon semifinal was crushing, he was a point from being up double break in the 5th set, Rafter's ball goes a foot out, and somehow the ump missed it, Rafter ended up holding and coming back to win the 5th.
Then Rafter lost to Goran in that great final.
That was Agassi's best shot for a 2nd Wimbledon.
Then that one French Open quarterfinal match I think against Grosjean when Agassi was up, Bill Clinton comes into watch part of the match, and he starts playing poorly.
Of course the best match he was ever a part of was the 01 US Open quarters against Sampras, 4 tiebreakers, wish we could've seen a 5th set there.
Then you throw in the whole doing meth at 27, which should've been his peak season athletically.
8 Grand Slams and a Gold Medal is still tremendous, but as an Agassi fan, I can't help the feeling that he should've ended up with at least 10 slams.
4 Aussie's make sense, to only have 2 US Open titles given that hard court was his best surface is annoying, lost so many semis and finals at the US Open.