He lost the ball match in S1 against a bulky opponent because he underestimated the the latter. While he's one of the top contenders now (even placing 1st in the first round), I just thought that the way he said he wants to prove that he's actually strong & skillful, but then proceeded picking the young K-pop idol Jang Jun as his opponent in the ball battle, because he thought idols are weak, is pretty low.
That ended up proving him NOTHING, he just wanted to win (which isn't wrong tbh & the choice was strategic, but then he shouldn't say anything like proving himself to anybody, lol). If anything, Jang Jun put up a good fight & despite being in a disadvantage, there were times when he almost got the ball & he wasn't a pushover at all.