It's painfully obvious in AC that Sephiroth was toying with Cloud the entire time, the reason Cloud got so tired when Sephiroth wasn't weary at all was because Sephiroth was putting barely any effort into the fight, compared to Cloud who was desperately keeping up.
He used only one hand when Cloud was dual wielding, and when Cloud was glowing blue, and of course when he turned Cloud into his personal pincushion.
That glowing/pincushion scene alone is visual evidence that Sephy wasn't giving anything close to his all.