The Phoenix Sect and the Ji Family were already locked in combat.
The Ji Family, by all accounts, shouldn't have been a match for the Phoenix Sect, but Ji Fengyun's arrival—with some unknown method—allowed them to break through their own limits, or perhaps the limits of the human body, substantially increasing their strength.
Of course, such an increase in power couldn't last forever, but the fact that Ji Fengyun had achieved this spoke volumes about his formidable nature and value.
During the first challenge in the ancient battlefield, Ji Fengyun could have participated, but the Ji Family didn't dispatch him; to them, Ji Fengyun was a treasure that couldn't afford any loss.
On the Phoenix Sect's side, they weren't at a disadvantage at all. The Ji Family disciples, despite breaking their limits, were at best, only on par with them.