I reckon every fighter thinks they are unbeatable, Tszyu was no exception.
If any fighter walks into the ring thinking they cannot win then that will happen, did Buster Douglas believe he couldn't beat Tyson? Tyson was unbeatable at the time according to the majority of experts and Kapow!
Tszyu thought he couldn't lose against Hatton and was wrong that's all, i've always believed he underestimated Hatton and paid the penalty for it, Hatton is a bloody good fighter and deserved his win.
As for Phillips, he got lucky and caught Tszyu at the right time, Tszyu was ill days before that fight and should have postponed it. A rematch would have been a waste of time as Tszyu would have won easily.
Tszyu just got old and lost to the hungrier, younger fighter in Hatton. He thought he couldn't be beaten just as Jones Jr thought Tarver couldn't beat him.
Mayweather firmly believes he is unbeatable, time will tell if he is right or not!