The imp jumped Jack the moment it had it had the chance thanks to his superior recovery rate.
Having a fight against a human was proving it a good amount of experience and helping it improve rapidly in combat, slowly turning the table on Jack who was just lucky enough to keep up with the pace.
The battle raged on and proved helpful to Jack as it was to his opponent, he was able to dodge and avoid the attacks without wasting too much stamina, allowing him to last more as time went by.
Soon enough, Jack was intentionally hit in his private already once again due to his carelessness as he failed to notice that the imp had kept track of what hurt him the most and eventually repeated it the moment he felt like he was losing the fight.
Every time it happened, it brought them to step one once again as the imp swiftly healed his injuries while Jack recovered from the pain.