Telemachus
Starter
I think those three guys are the embodiment of what Bilas is espousing. The toughness is the state of mind, and the focus to execute the fundamental correct play under pressure and duress.
I agree that they all had those qualities, it was their never back down attitude that made them tough. A player like Hamilton plays fundamentally sound, and plays well under pressure, but I wouldn't call him tough. He gets pushed around too much IMO, while the other 3 I mentioned always pushed back and made thier presense felt.