Zhou Tian was indeed weakened in this world. If he could fully leverage his strength in the Outside World, then here he could only exert about seventy to eighty percent of his Combat Power.
However, compared to Zhou Tian, Pan Shi wasn't doing much better.
Indeed, despite being in this world, following the Power System Pan Shi practiced, his Attack Power would not only be unweakened but arguably even stronger than when he was in the Outside World.
However, as far as Zhou Tian knew, Ascenders like Pan Shi weren't allowed to return. According to general theories, since Pan Shi had ascended from this world due to his strength reaching a certain level, naturally, if Pan Shi still held the strength of an Ascender, it would have been impossible for him to return to this world. Even if Pan Shi did return, according to the World Rules, he would have already been forcibly expelled.